I sort of like both ideas, heres my opinion on both.
-Keeping it white will in a way promote equality, which is good and needed, but on the other hand will also force on clones "having" to wear certain attachments in order to display their rank. It gives out attachment motivation but takes away the pattern motivation, making the whole clone look not really change a lot through the whole IC char development.
-Making it a single legion or picking a neutral color for it, will keep things as they are now, which aint good or bad. As for attachments, like Vash said, should represent specialization.
And heres my idea, taking a little bit of both:
Considering our strengh and hopes comes mostly from the new blood and knowing our numbers are not yet big enough to support two legions, why not make every clone part of the 501st, leaving white armor for our NPC's? The idea is motivating and giving tools to both elder and new clones to "command" and deal with whatever situation they have to face. But, how can it be done? Simple, a ceremony, not really killing any legion IC, but having the Commander move "the very best of GAR" (everyone of us) to the 501st. If things keep getting better in the future (which they will if we aim at it) 501st will no longer be needed, as we will have quantity over quality again. No elitism, no exclusive gear, everyone equal and yet "the very best of GAR".
Having two legions has been pointless for more than an year at least so im up for whatever you all decide on that changes that.