[Not that such behavior was new to Tohko. After all, she had dealt with chessmasters in the Tower before, and Wilhelm was worse than any chessmastery normal humans could cook up.

In her opinion, anyway.]

There are a few theories running around about them. The best one, in my opinion, is that they keep your soul in your body and from rejecting it. Like you would take care of an organ transplant, if you're aware of those.

As for why some have lighter colors and others don't..darker colors indicate a stronger liquid to keep the soul in the body. You'll see that with beings of the collective unconscious, angels, that sort of things.

However, lighter colors mean that the liquid to keep in your soul is pretty weak. There's not much needed to keep you in.