Well I might want to roll GGGR with focus (or RRR with explosive munitions for another example).But I also don't see any issue in restricting the dice you actually roll to maximum of 2 of each color, because you can always choose just 1 green die into your attack pool if you're focused.
If you take it that literally, then you can't roll RRR, but you could take RRR and then use Heavy Firepower to remove an R and add 3 damage... So you'd be rolling only 2 reds...