question: Why does black ink often appear a metallic purple/orange colour?
comment: Because black is all of the colors. It has purple and orange in it. So depending how the light hits it depends what shines
passages: Colored inks are also available, where the ink is visible in normal light (as with a regular tattoo) and the ink will glow vividly under UV light. Due to the mixing of visible and UV pigments the resulting color is not as vibrant in either lighting situation as a dedicated ink.
   An application of leuco dyes is in the Duracell battery state indicators. A layer of a leuco dye is applied on a resistive strip to indicate its heating, thus gauging the amount of current the battery is able to supply. The strip is triangular-shaped, changing its resistance along its length, therefore heating up a proportionally long segment with the amount of current flowing through it. The length of the segment above the threshold temperature for the leuco dye then becomes colored.
   Optically variable ink
  
  Optically variable ink (OVI) also called color shifting ink is an anti-counterfeiting measure used on many major modern banknotes, as well as on other official documents (professional licenses, for example).
  
  The ink displays two distinct colors depending on the angle the bill is viewed at. The United States fifty-dollar bill, for example, uses color shifting ink for the numeral 50 so that it displays copper at one angle and bright green in another.
   Damage to the compounds in tattoo ink cause the color of the resulting tattoo to change over time. For UV tattoos this may mean the tattoo becomes more visible under visible light or may not glow in black lighting. Blue UV inks are known to yellow or turn slightly brown with sun exposure.
  
  Section::::Safety concerns.
   Section::::Organic materials.:Leuco dyes.:Inks.
   In inkjet color printing and typical mass production photomechanical printing processes, a black ink K (Key) component is included, resulting in the CMYK color model. The black ink serves to cover unwanted tints in dark areas of the printed image, which result from the imperfect transparency of commercially practical CMY inks; to improve image sharpness, which tends to be degraded by imperfect registration of the three color elements; and to reduce or eliminate consumption of the more expensive color inks where only black or gray is required.
   Other inks work in a near opposite way by "absorbing" ultraviolet light but without fluoresceing. When these are used on fluorescent paper, the inked areas fluoresce less than the surrounding paper area when under an ultraviolet lamp. This is especially a property of inks with a yellow tint.
  
  Some UV-visible inks may be detected on a photocopy, due to the relatively strong ultraviolet component in light from the photocopier scanning head.
  
  Examples of inks revealed by ultraviolet light are:
  
  BULLET::::- Laundry detergents containing optical brighteners
  
  BULLET::::- Soap
  
  BULLET::::- Body fluids, serum, saliva
  
  BULLET::::- Sunscreen
  
  BULLET::::- Lemon juice
   For some inkjet printers, monochrome ink sets are available either from the printer manufacturer or from third-party suppliers. These allow the inkjet printer to compete with the silver-based photographic papers traditionally used in black-and-white photography, and provide the same range of tones: neutral, "warm" or "cold". When switching between full-color and monochrome ink sets, it is necessary to flush out the old ink from the print head with a cleaning cartridge. Special software or at least a modified device driver are usually required, to deal with the different color mapping.
   Color bleeding (printing)
  
  In printing and graphic arts, mixing of two dissimilar colors in two adjacent printed dots before they dry and absorb in substrate is referred to as color bleeding. Unless it is done for effect, color bleeding reduces print quality.
  
  Prior art applied this term to the phenomenon of single color ink following the fibers of the paper.
   Printed in the smaller point sizes used in telephone directories, the ink traps are not visible, having done their job; filling in and smoothing out the character stroke. However at larger point sizes, and on coated paper stock there is not enough ink spread to fill in the traps and the shape of the traps remain noticeable. 
   Dye-based inks are generally much stronger than pigment-based inks and can produce much more color of a given density per unit of mass. However, because dyes are dissolved in the liquid phase, they have a tendency to soak into paper, making the ink less efficient and potentially allowing the ink to bleed at the edges of an image.
   Studies of various tattoo pigments have shown that a number of pigments (most containing iron oxide or titanium dioxide) change color when irradiated with Q-switched laser energy. Some tattoo colors including flesh tones, light red, white, peach and light brown containing pigments as well as some green and blue tattoo pigments, changed to black when irradiated with Q-switched laser pulses. The resulting gray-black color may require more treatments to remove. If tattoo darkening does occur, after 8 weeks the newly darkened tattoo can be treated as if it were black pigment.
   Section::::Invisible ink types.:Inks which alter the surface of paper.
  
  This includes virtually all invisible inks, but pure distilled water can also be used in this way. Application of any fluid will alter the paper surface fibers or sizing.
  
  Fumes created from heating iodine crystals will develop the writing, which will appear brown because the iodine sticks preferentially to the altered areas of the paper. Exposing the paper to strong sunlight will return the writing to its invisible state, as will using a bleach solution.
   Use on packaging can be to detect temperature history during shipping and to indicate proper heating in an oven.
  
  Section::::Examples.
   There are several types of thermal printers, among them and marketed at present time as "Tattoo transfer copiers", there are two main types made by different manufactures: Thermo-head type and Heat lamp type
  
  Section::::Thermo-head type.
  
  The black parts on the original sheet, once scanned, make the copier activate the heating elements that produce some chemical reactions on the "thermo-sensitive" copy paper that darkens its surface
   The ink is released from the ink sacs (located between the gills) and is dispersed more widely when its release is accompanied by a jet of water from the siphon. Its dark colour is caused by its main constituent, melanin. Each species of cephalopod produces slightly differently coloured inks; generally, octopuses produce black ink, squid ink is blue-black, and cuttlefish ink is a shade of brown.
  
  A number of other aquatic molluscs have similar responses to attack, including the gastropod clade known as sea hares.
  
  Section::::Inking behaviours.
   With current ink technology, the total CMYK ink in the shadows refuses to stick after it reaches the dark shadows (usually above a 250% total CMYK coverage), and begins to peel off. To prevent this, printers developed UCR, in which neutral shadows – which would have normally been produced by overprinting the four inks "Cyan, Magenta, Yellow and Black" on top of each other (high ink coverage) – are replaced with the single layer of "Black". UCR removes the color inks "under" the Black, resulting in a single layer of ink which sticks to the sheet better, and saves on the consumption of ink.
   where "R"("λ") is the reflectance of ink combination "i", and "w" is the relative proportions of the 16 colors in a uniformly colored patch. The weights are dependent on the halftone pattern and possibly subject to various forms of dot gain.
  
  Light can interact with the paper and ink in more complex ways. The Yule–Nielsen correction takes into account light entering through blank regions and re-emerging through ink:
   By the 1970s, because of the advent of psychedelic art, artists became used to brighter pigments, and pigments called "purple" or "bright purple" that are the pigment equivalent of became available in artists pigments and colored pencils. Reproducing electric purple in pigment requires adding some white pigment and a small amount of blue pigment to red-violet pigment. Even then, the reproduction will not be exact because it is impossible for pigment colors to be so bright as colors displayed on a computer. 
  
  Section::::Variations of red-violet.
  
  Section::::Variations of red-violet.:Kobi.
  
  At the right is displayed the color kobi.
   Thermochromic ink
  
  Thermochromic ink (also called thermochromatic ink) is a type of dye that changes color when temperatures increase or decrease. Often used in the manufacture of many toys or product packaging, as well as thermometers. Thermochromic ink can also turn transparent when heat is applied; an example of this type of thermochromic ink is found on corners of an examination mark sheet. This proves that the sheet has not been edited or photocopied, and also on certain pizza boxes to show the temperature of the product.
   When a very dark area is desirable, a colored or gray CMY "bedding" is applied first, then a full black layer is applied on top, making a rich, deep black; this is called "rich black". A black made with just CMY inks is sometimes called a "composite black".
   UV inks do not blend during application, as normal inks do. Additionally their consistency is thinner. Only experienced tattooists should apply UV tattoos, and should have a blacklight available to help check their work especially if tattooing with an ink intended to be invisible under normal lighting conditions. Due to these complications UV inking takes longer than when using conventional inks.
  
  UV tattoo ink's fluorescence will be dulled if regular ink is used on top of it. Therefore, for vibrant high impact tattoos normal ink is used, allowed to heal, and then highlighted with UV inks.
  
  Section::::Photochromatic tattoos.
   Magenta
  
  Magenta () is a color that is variously defined as purplish-red, reddish-purple or mauvish-crimson. On color wheels of the RGB (additive) and CMY (subtractive) color models, it is located midway between red and blue. It is one of the four colors of ink used in color printing by an inkjet printer, along with yellow, black, and cyan, to make all the other colors. The tone of magenta used in printing is called "printer's magenta".
   A typical rich black mixture might be 100% black, 50% of each of the other three inks. Other percentages are used to achieve specific results, for example 100% black with 70% cyan (C), 35% magenta (M), and 40% yellow (Y) is used to achieve "cool" black. "Warm Black" is 35%C, 60%M, 60%Y, and 100%K. The colored ink under the black ink makes a "richer" result; the additional inks absorb more light, resulting in a closer approximation of true black. While, in theory, an even richer black can be made by using 100% of each of the four inks, in practice, the amount of non-black ink added is limited by the wetness that the paper and printing process can handle. (A safe and practical rule of thumb is that ink coverage should not exceed 240% on normal papers. Papers that "pick", such as low-end recycled papers, should have even less coverage.) Wetness is not a problem with laser printers, however, and registration black (or "400% black") produces very striking results in laser prints. Interesting effects can also be achieved with a laser printer by combining 100% black and 100% of cyan, magenta, or yellow.
   OVI is particularly useful as an anti-counterfeiting measure as it is not widely available. One major manufacturer is a Swiss company called SICPA (Société Industrielle et Commerciale de Produits pour l'Agriculture). Additional suppliers include Sun Chemical (through their Brand Protection Division based in Manchester, UK), the Brazilian company Sellerink, located in São Paulo, Brazil and the Swiss company Printcolor Screen AG, located in Berikon, Switzerland.
  