Use a string client tag (you can’t use anything else).
I use HEX, but really it doesn’t matter, I guess whatever’s more comfortable.
FYI if you bi-directionally bind the client tag to a fillPaint property, re-assigning the colour by picking a new colour from the colour picker on the fillPaint will set it to the string
"color(r,g,b)" (assuming you don’t force the colour of the client tag using an expression - which you should definitely do to avoid this very situation and where someone forgets, e.g. me, that it’s bound and accidentally changes the colour everywhere )
“Initialise” it via an expression in the client tag, as above