I don't know if it already exists, i briefly searched for it and i didn't find anything. What i suggest is a command-based color customization system pretty much like /color <name> <RGB> but not for items but for the colors themselves.
Exemple:
/setrgb 4 $rgb(0,0,0)
This would change the color #4 (CTRL+K 4) to black (RGB 0,0,0).
