My bay is iridescent and I have met a buckskin QH stallion that was SO iridescent it wasn't even funny. The stallion was iridescent even in the shade. Monty isn't, so I consider him to be "just a bay".
I think it could well exist, and from my experience, not only in akhal-tekes. But it's not present in my horses. My foal, I don't know, she's still fluffy and not shiny at all yet, but what's shed out, though shiny and somewhat iridescent in the sun, is nothing on that stallion I met. He was gorgeous.