My take on the Phoenix.

I tried to stick to a color scheme that matched the world... in short, I wanted a "shimmering reddish iridescent rainbow" look, heh. It sounds like the Phoenix has rainbow feathers, but I wanted a call-back to the typical red coloring of a Phoenix.

Iridescence is a challenge... especially since it changes based on your perspective. =P But I think I'm happy with my results. I changed my approach a little from my original test iridescent effect (King's Man's plume), so I think this works better. I came up with a set of simple rules to follow to approach my iridescence. Since it's basically the light spectrum, I came up with the darker tones as base... Red to blue/purple, and saved the middle of the light spectrum for highlights. From what I saw in reference, iridescent red surfaces highlight up to orange->yellow/green (middle of the spectrum) while blue or purple iridescent surfaces bend back towards green... with maybe a hint of red. So for my larger, blue/purple feathers, I went towards green while throwing in a tinge of the base coat red in random feathers to tweak the spectrum in that area too.

I then tried to maximize contrast to get a shiny, shimmery look to the feathers... went all the way up to pure white for most feathers... but the super bright highlights also helped me sell the iridescent effect... at least I think so. =)