ok it took a bit of thinking and playing around with the colours, but I'm back with my thoughts.
here I've done my best to try and make them distinct: blue-green for commons, yellow-orange for uncommon, red-purple for rare, with deep blue/purple for the OMGs.
(I've used super instead of extremely and U for uncommon instead of UC, as I explained in my previous post.)
the biggest change is orange moving from rare to the uncommon group and very rare becoming magenta, but other than that I've tried my best to keep it as close to current as possible.
however to me it seems there aren't really enough unique colours to make each rarity stand out. yes, you can fill them all, but a lot of the in-betweens are very similar to their neighbours. so while seeing them as a whole works fine, it falls apart when seen individually as many times it can be hard to tell which colour is what.
take this example; is that orange a very uncommon or a super uncommon? what about the green, is that common or very common?

so in all honestly, the colours would work a lot better and be less blended if we removed the OMG tiers.

I like these colours a LOT better, it's more clear and each colour is very hard to mistake for another. sure it does change a lot in the rare group, but if we want to keep a rainbow theme this is kind of the way it has to be. either way this spectrum right here has my vote!
but we can't just ignore the OMG tiers, I feel we
need them to make trading better so removing them doesn't seem like the best action just for the sake of colours. maybe we can do something else with them, keep the same colour as their neighbour but make them extra special, like making them shiny?

or making the rare prismatic and the common greyscale. just an example but I honestly feel something like this, with the OMGs being special, would be the best approach.