BTW, I'm assuming in my response that:
Set Field[ColorChoice($$OnRep), Color]
isn't exactly what you see in the script editor. correct syntax would be:
SetField [YourTable::ColorChoice [$$OnRep] ; YourTable::Color[$$onRep] ]
Haven't used repeating fields for this type of thing in years so I may be wrong, you may have to use the getRepetition function if that syntax is not permitted:
SetField [YourTable::ColorChoice [$$OnRep] ; Get Repetition ( YourTable::Color ; $$onRep) ]
I also suggest you not use pair of repeating fields for this in the first place. A related table where you have 10 related records is a much more flexible approach for doing this. Among many other advantages, you'll no longer need to keep track of the active repetition.