Well, my Orks main fluff is that they're a grouping together of disparate clans as brought under the boot of Ghazghkull Thraka.

My Aeldari are painted in autumn colors and markings as one big "fall of the Eldar" pun.

I have a group of T'au who are scouts and fighters on a fringe, with some scrappy resources and having to have merged units together from Auxilliaries. Galg (and an unnamed plant based species) take on the role of Stealth Suits, Demiurg and Gue'vesa make up half the pathfinders, but of course it's homeworld original T'au Sept T'au in charge.

Then there are these groups of bare ceramite clad warriors whose special doctrines involve obfuscating their leaders (by not marking them out with any special colors like other chapters would their Sgts) and quick adaptive weapon swaps in battle. Their homeworld is Planus Noctis which is renowned for its stretches of featureless landscapes.