\unexpanded\def\colored[#1]% {\groupedcommand {\definecolor[@colored@][#1]\doactivatecolor{@colored@}} {}}
i'll add \colored to the core (this is the mkiv definition)
Concerning this specific issue: I often miss \color[r=.5,g=.5]{something orange} in mkii as well. Is the "command space" in the definition of \color command itself already polluted, so that this is not possible? \color is very complex command, so there's probably a chance that "if argument are assignments, then define the color first and then use it, if it's not assignment, use the already known color" would not work. Is there any specific reason why only mark iv (apart from the fact that you consider mkii "frozen")? Mojca (I have some more questions concerning this thread.)