Color is a valuable tool that can be used by the user for various personal purposes. In Corkboard mode, text color is disabled along with all the other text attribute change functions. The Scrivener Way or mantra should be “no limits”. Having no limits affords more power to the user. My suggestion is to open up all text attributes and text formatting in Corkboard mode.
AIUI the Synopsis (which is what the Corkboard text is) is a plain text file behind the scenes. You can see this if you examine a Project file – the document files are numbered RTF files. If document 123.rtf has a synopsis you’ll also see a 123_synopsis.txt file. That means that colour attributes etc aren’t available because the txt file format doesn’t allow it.
I don’t know the design decision behind using .txt files (space? performance) and the implications of changing it.