Originally Posted by lunchbox (from BD)

It's a cabezon. I get those all the time at work (I study baby fish). Even at that size the color is really variable - I've seen a bunch like that, some red, some olive, some brown, some almost all black, some bright green/black/silver like mackerel. usually the bright silver ones are the smallest and they darken up as they get a little bigger (bigger = ~50 mm = ~2 inches).

Originally Posted by lunchbox

Cabezon, like most fish, spend the first part of their lives as larvae floating around the open ocean with other zooplankton. After a few days, weeks, or months (depending on the species), the lucky ones that end up near a reef settle out and grow up into adults. Cabezon do this when they are about 2 inches long. Several of our local species (cabbies, some nearshore rockfish, calicos, etc.) like to settle out into the canopy of the kelp bed, where they hang out for a few weeks before heading down to the bottom. These are the species that I work with.

Disclaimer: The following is pure speculation, with no data to back it up.
I suspect the reason the little cabbies < ~2 inches long are silver with green or blue backs and black markings is to camouflage the fish in open water (think mackerel). It's very common among fishes to have clear or silvery/white larvae for this exact reason. Once the cabezon gets onto a reef it will darken up to hide against the darker background of the reef, and that's where you get the olives, browns, and reds starting to show. I don't know why some are blue or green. That was discussed on BD about a year ago, so you can search for it if you want.