Why isn't any color showing up in my .3mf file in Windows 10 3D Builder So, I made a 3D picture of a cute mouse using Microsoft Paint 3D and exported it as a .3mf.   

Why didn't the colors match when I opened the file in Microsoft 3D Builder?
Per the 3MF Specification, colors in the .3mf files are sRGB.

If you are looking at gross color defects, I would look a the raw codes generated in the 3mf to make sure they make sense.

If you are talking about subtle color differences, that is much more complicated.

Color have always been a tough thing to define as it is affected by whether the it is emissive (like displayed on a screen) or reflective (like a physical object).  The way it is perceived is affected by ambient light, surface texture and more.  I worked on a project once where Marketing wanted all the colors to match (on the screen, on paper, on plastic, on on fabric) and it was a mess.