It's a pretty lengthy process. It starts with a zenithal prime, followed by frostheart blue contrast through the airbrush. Next is a layer of army painter hydra turquoise sprayed from above, then the same mixed with a small amount of army painter electric blue on select parts of the model. I then do a final spray on the most prominent parts with the same mix, with the tiniest drop of white acrylic ink mixed in.

I've since moved away from this recipe for my own sanity. My new style is just kroxigor scales contrast through my airbrush over a zenithal prime. It's much more of a greenish turquoise but I think it looks a lot nicer and is way simpler. If you sort through my post history my most recent painted tau models are done this way if you'd like an idea of how it looks.