Why isn't any color showing up in my .3mf file in Windows 10 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.