It’s nice to have keyboard shortcuts for different colors, but it’s very likely that one will tend to use a limited palette in a notebook to help organizing.
So my idea is: Keep tract of the colors defined/used in the project and allow me to quickly toggle through them with some simple key combination.
I believe that individual keyboard shortcuts for different colors would then be unnecessary and valuable keyboard shortcut space would be released.
The workflow would be extremely simple, because you just define a color once and then go on without remembering anything new.