Why is Green Martyrdom green? I'd heard of green martyrdom a few times on the radio, but never gave it much thought

Three kinds of martyrdom that are counted as a cross to man, i.e. white martyrdom, green martyrdom, and red martyrdom.—The Cambray Homily (Thesaurus Palæohibernicus, II., p. 246).

Green martyrdom somewhat anachronistically seems to have been defined as having something to do with dollar bills being of a greenish hue.  I don't think that would be the case in the Ireland when the Cambray Homily was transcribed.
So why green?  Red and White I understand, but green usually has a different significance than dying.
Why is ”green martyrdom” green?
A green martyrdom is a type of spiritual mortification or martyrdom that the faithful may perform during the entirety of their life without shedding their blood or fleeing persecution to live an ascetic life style such as the Desert Fathers. Green is the color most commonly associated in Europe and the United States with nature, vivacity and life.
If someone were to say that it is simply because this terminology originated in Ireland and green is the colour of the Irish or leprechauns, I would say that symbolism is all wrong.

The modern image of the leprechaun sitting on a toadstool, having a red beard and green hat, etc. is clearly a more modern invention, or borrowed from other strands of European folklore. The most likely explanation for the modern day Leprechaun appearance is that green is a traditional national Irish color dating back as far as 1642.

The original colour of Ireland was blue, not green!

In reality, the authentic color for St Patrick, the patron saint of Ireland, is blue. It can still be seen today on Ireland's Presidential Standard and other ancient Irish flags. In the 1700s, green replaced blue and became the official color for Ireland. It is one of the three colors of the modern Irish flag. Green seems like a more logical choice when you consider that Ireland is known as the Emerald Isle because of its beautiful green countryside. The expression &quot;the wearing of the Green&quot; refers to both the shamrock that St. Patrick used to teach the Trinity and the bright green uniforms worn by soldiers during the Irish Rebellion of 1798. Green has also been the color that represents support for the Irish dream of independence.
The green of the Irish flag is the official color for St. Patrick's Day. It is a spring green with less blue in it than some of the other shades. - The Official Colors of St. Patrick's Day

So why is green martyrdom green?
First of all let us look at the symbolic colours of martyrdom which actually were mention by Pope Gregory the Great at a time in history that the Celtic Rite was very much alive within the Church. This is the same pope that called the Anglo-Saxons Angels. The story goes that Pope Gregory the Great saw some fair-haired and fair-skinned slaves in a slave market in Italy, and was told that they were Angles. “Not Angles but angels,” he replied.

Degrees of martyrdom
Some Roman Catholic writers (such as Thomas Cahill) continue to use a system of degrees of martyrdom that was developed in early Christianity. Some of these degrees bestow the title of martyr on those who sacrifice large elements of their lives alongside those who sacrifice life itself. These degrees were mentioned by Pope Gregory I in Homilia in Evangelia, he wrote of &quot;three modes of martyrdom, designated by the colors, red, blue (or green), and white&quot;. A believer was bestowed the title of red martyr due to either torture or violent death by religious persecution. The term &quot;white martyrdom&quot; was used by the Church Father Jerome, &quot;for those such as desert hermits who aspired to the condition of martyrdom through strict asceticism&quot;. Blue (or green) martyrdom &quot;involves the denial of desires, as through fasting and penitent labors without necessarily implying a journey or complete withdrawal from life&quot;.
Also along these lines are the terms &quot;wet martyr&quot; (a person who has shed blood or been executed for the faith) and &quot;dry martyr&quot; which is a person who &quot;had suffered every indignity and cruelty&quot; but not shed blood, nor suffered execution. - Christian martyr (Wikipedia)

Thus a green martyrdom &quot;involves the denial of desires, as through fasting and penitent labors without necessarily implying a journey or complete withdrawal from life*”.
From the Celtic understanding, adopted through out Christian lands, there are three types of martyrdom:

Red Martyrdom - those who die for the sake of the faith.
White Martyrdom - those who leave behind all possessions, titles, and relationships for the sake of the Kingdom of God. This includes hermits, anchorites, and most monastics.
Green Martyrdom - Living the ascetic life in the place and position where God placed you. It &quot;...consists in this, that by means of fasting and labour he frees himself from his evil desires, or suffers toil in penance and repentance.&quot;

Green is the color of new vegetation and the most wide spread colour in nature. It symbolizes the hope of new life which is ours in the life of Jesus Christ. Because green martyrs practiced forms of penance without necessarily implying a journey or complete withdrawal from daily life of the Christian community, it is fitting that their form of martyrdom would be green!
The green martyrdom was a way to sacrifice when lives were no longer in danger.

The Irish monks in their ingenuity thought up another form of martyrdom that did not require persecution and bloodshed: they would come to call themselves, “Green Martyrs” and give their whole lives dedicated to worship and prayer.
The Green Martyr was one who followed the cost of discipleship that Jesus laid down, the “deny yourself, pick up your cross, and follow me” kind of commitment that leads to a self-sacrificial existence. It is in this sense that the symbolic colour of green should be understood.
They chose the most remote corners of the Island, places like Skellig Michael, a rocky outcropping some 8 miles southwest of the tip of the Irish coast. Literally, “the ends of the earth” as their maps simply did not go any further than this point.
Perhaps without realizing it, all of these modern movements have their roots in the ancient “Green Martyrs” of Ireland!
So here are 3 possible applications to becoming a modern “Green Martyr” (without having to move to Skellig Michael in Ireland):

Cultivate a lifestyle of radical commitment to God in prayer.

You can learn to cultivate a daily, hour by hour dependence upon the Spirit by new levels of self-control and self-discipline. Pray more! Pray fervently. Fast &amp; pray, too. Try checking in with the Holy Spirit every hour of the day.

Read &amp; meditate on scripture, daily.

The monks did. I visited the ruins of Nendrum Monastery, between Belfast and Downpatrick in Ulster, Northern Ireland. It contains the best-preserved ancient sundial in Ireland, where I noticed there were 8 markers which marked 8 distinct times of the day that these men and women of God would pause in their lives, pray, and meditate on Scripture. Try this: set some markers on your mobile phone to alert you to pause and pray throughout your day!

Find a spot in your home to dedicate as “holy space” to God.

The Christian Celts called these places, “thin places” because the veil between Heaven and Earth was so thin and they could sense the glory and presence of God lingering there. This could be a prayer room, or even a favorite chair where you commit to daily communing with the Spirit of God, interceding for others, or just reading Scripture. And the more time you spend in communion with God, the more you personally carry a “thin place” with you, wherever you travel, and can bring the presence of the King of Kings to others who need Him. - The Green Martyrs of Ireland at the Ends of the Earth

The following articles may be of interest to some:

Not just red anymore: the colors of martyrdom
Friday Fact: Red, White, and Green Martyrdom