But, beware - as the colour-meister - that if you want to make a change to improve the game in general, that there are many factors involved:
Such as it must work for each scenario - minimally that colours do not *clash* for all the starting countries. Then, there are the issues of avoiding colour clashes among the King/Duke/County heirarchy. And, of course, neighboring King/Duke/County heirarchies. Not withstanding non-playable non-Christian countries vs the Christian titles that can be created.
In short, the whole thing is a bloody mess - but, I suspect that you just want to use a colour that you like, rather than whatever is currently assigned in the game files.
[one last point, do not pay any attention to the names of the colours - they are arbitrary and do not necessarily represent the actual colour displayed on the realm map...]