Quest is asking for a blue symurgh. I have one (Iridescent) but she's not showing up. Is it because she has a custom skin?

Yup, custom skins replace colours so they don't work for quests. :(

Darn... I'm aiming to collect as many skins as possible, so the beasts becoming un-questable is a shame. Oh well, thanks for the answer! Time to get some spares.

This is why my black gp 3 unicorns don't have Monues's beautiful black Friesian skin on them, hehe. Pity. There's a major gold quest that they can fulfil, you see.

Yep, just got that gold quest again today. This otherwise-excellent black unicorn will never be getting a fancy skin, poor girl. x)

Oh, this might be a silly question, but do deviant versions of colours work in quests? Like a Black* uni, or (to try later) an owl or snow leopard gryph?

That's a good question, I can't say I've ever had a deviant fulfil a colour quest, so I couldn't tell you...