Hello - in plain Rhino, there are two things you can do, that I can think of anyway - one is to modify, ‘by hand’ the colors/txt file here:
C:\Users\Pascal\AppData\Roaming\McNeel\Rhinoceros\5.0\Localization\en-US\Support
C:\Users\Pascal\AppData\Roaming\McNeel\Rhinoceros\6.0\Localization\en-US\Support
This file is what Rhino reads to put colors in the list on the color picker. Yes, it should be easier but that’s how it is…
The other is to save single color textures as .rtex files from the texture panel. These can then me used as colors that will affect any material that uses them - i.e. you can edit the color of multiple materials from a single texture.
-Pascal