I'd guess it's supposed to be yellow like always, just Forgeworld have gone for the weathered, gritty look by having the Iron Warriors look a bit dirty and grimy. Maybe try painting the yellow and then giving them plenty of black/brown washes to fade and dirty it up.
Now, what I've always wanted to know about painting Iron Warriors is how to get
this look;
Now that ^ is badass. I can't even really describe it, it's like a dirty, grimy metallic greenish colour. All the "how to" videos and painting guides for
IW just seem to be gunmetal grey with a black wash thrown in. I assume you'd get the above look with heavy use of multiple glazes and washes, maybe over a glossy metallic? Similar to the Forgeworld recipe for Alpha Legion? But I'm not sure.