They just want to fool the birds. ZSTs for example have at least 4 different colors of larva. Once a bird patterns food it will search for that color. They also have green and brown pupa, but I have had a spring brood green pupa wait until the next spring to emerge.

Sometimes BSTs will take the color of what they pupate next too with variations from whitish to greenish to brownish.

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One of my BST caterpillars became my first brown chrysalis this morning (all the others I've seen before were green). Actually he looks about the same color of the gray leafless stick he pupated on, which isn't exactly brown...he's more of a dark grayish color, so I'm not sure brown is really the right description. It also seems a bit small for something a butterfly is supposed to come out of, but it's been a while since I've seen a BST chrysalis, so my size expectations may be a bit distorted, and he may be normal-sized for all I know.

This is the first year that all my BST crysalis' are green. There are usually more brown with a few green. Weird, isn't it?

It makes sense to mix up the chrysalis color like Runmeded said. Love those ZSTs by the way--hope someday they come back to this area of IL.

I found a nice blog that explains the coloration better.

Here is a link that might be useful: Variations on Theme Chrysalis Color

I now have two brown ones. The brown ones are on white netting I use to protect them. But there were three green ones on the white netting too. Weird.