What's the most under-represented colour in Warhammer 40k, and how do we fix it?

All my corsairs got Orange Hairs for that very reason

Fire Dragons have orange armor, and the studio paint scheme for the Corsair Eldar is orange-ish (might also be a very dark yellow, but I say it's close enough). It is a very rare color, though.

Octarine

No, definitely not. Why would you even think that? are you retarded?

Don't tau have primarily orange equipment?

They made it white now.

You're all forgetting about purple. I never see orks in purple.

Mahogany

Came here to post this.

I accidentally made orange-ish marines once when I painted Averland sunset over black primer, and it hit me that I'd never seen orange represented in 40k.

I'm having difficulty figuring out a scheme that would work well with it. Maybe dark silver and a kind of light blue?

Tau's equivalent of Ultrasmurfs (it's not the "official" uniform colour scheme, just the one GW used as representative) used to be "Ochre", but that's more like an orange-tinted brown than a legitimate orange in its own right.

Soul Drinkers, Slaaneshi, demons of Tzeentch and sometimes Deldars