I think our organization allows either system....resident sensei preference. As far as I know the ones that use color use the the same ones.
One of my students visited another dojo in our org. that uses the same color system. My student was wearing his purple belt. One of the junior students at the dojo he was visiting asked what style of aikido he did. When he answered Seidokan (the same as them) the junior replied "there aren't any purple belts in Seidokan". It turns out that dojo hadn't had anybody at yonkyu or sankyu rank since the junior had been training