I get 1-2 of these each day I bother playing 3-4 Arena matches. The rest are majority blue but a few green also drop.

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I think I have most ease killing the Golden Skitterlings that drop these in 2nd, 3rd, and 4th Arena (in that order).

I have yet to get an Orange drop which I thought someone had said is a possibility, but I don't think so. Hey, at least we get some nice purple drops now and again.