That's a good question. But in my experience CA is not only purple, it's magenta, orange, green, and cyan. I'm a shallow DOF whore, and pretty much all I use is 1.4 to 2.2, so I have to deal with CA a lot, and it's definitely different colors.
I believe the reason is that light has a spectrum of colors that when mixed properly aren't always able to seen, and it's literally the bending of this light by the lens (like a prism in sunlight) that causes the colors to split apart. It's a sharp bend, a sharp contrast, so the colors are sharp. At least that's what I've understood over the years.
I'll also say that my 200/2 is my favorite lens, I shoot it exclusively at f2 has almost no CA, which is fantastic. Really, it blows every other lens away in that regard, and in most others.
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