Onions and rainbows (no, I don't mean Kershaw!)

[Daniel Moore]

I got to cut up a big white onion for dinner with my fixed blade. It's made out of 0-1, it's got a ferric chloride etch, and after I cut the onion, I noticed some REALLY intense looking rainbow action all over the blade. I also noticed that the knife was rainbowed a little bit when I re-etched it with ferric at one point. It comes off eventually, but does anyone know what this is exactly? Is it a reaction between oils on the blade and the acids or the steel and the acids etc?