Yellow does not appear ever on any MX datasheet I have seen. I also believe those are yellow stems from Kailh MX copies. Though the Alps mount don't have any listing in the official MX datasheets either.
The most obscure or difficult ones to find in my experience (since I still have zero) Super blue 160cN which are a dark navy blue color, clicky grey 105cN, orange which are basically red with a tiny keymount that is incompatible with MX caps.
My favorite uncommon MX switches I have in my collection are my linear white/clear type. Those are a little lighter feeling than blacks, but heavier than red. I could probably part with a couple.
I also have a bunch of what I think are M9 switch if you are interested in those.