Army colour themes

I'm collecting a few armies simultaneously, some deliberately as viable forces (like Tyranids, Imperial Guard, Space Marines and Necrons) and some just because I like the models (Tau and Orks), and I think it would be great if each army had its own "colour", so that they would look distinct both on the shelf and on the battlefield.

The first few are easy:

Tyranids = Kraken Red
Necrons = Silver/Black
Imperial Guard = Cadian Green

I'm not quite decided on the others yet. If I model my Space Marines as Ultramarines, then blue is the obvious choice, but I am leaning towards making my own Chapter, and am very tempted to go purple (not Hawk Lords, but something similar). Orks, I think I can get away with a mix of salvaged bits, rust and dirt. That should go well with their green skin. Tau, on the other hand, should look clean and striking - heavy weathering and battle damage just doesn't suit them. A maroon or burgundy, perhaps. There's a beautiful burgundy in the Tau Codex, but I can't find a guide on how to paint it. Then again, if I'm not doing Ultramarines, blue becomes free all of a sudden.

Of course, as a Lazy Painter, I would prefer to just use an unmixed GW paint, but I think that if I'm using enough paint, it's working cooking up a unique batch.

So for the rest of my armies:

Space Marines - Purple?
Orks - Rusty Dirty Brown?
Tau - Burgundy? Or Blue?

This will be a work in progress - I plan to finish my Tyranids first, then hopefully there will be a new Necron Codex, and I can work on them for a while. In the meantime, someone may come up with a good scheme that can inspire me, or maybe I'll have a brainspasm of original thought, you never know.