I have a Tau band calling themselves Hand of Ash

Their ethereal, Loresaw Whiteash, has a previously undiscovered genetic defect that has prevented them from being fully integrated into the Subliminal collective that pushes the greater good. They haven't rejected it, but they have made plans for independence should the opportunity arrive.

My Ork army is Blood Axes. Their warboss saw some humie game hunters wearing camouflage one time and decided it must be a good idea, so he set the grots to work making enough for the whole band. It turns out, yellow safety camouflage isn't super effective in most environments, but they're convinced it still makes them dead sneaky.

Raven Guard: my head canon is the chapter needs to venture out from Deliverance to find Corax, due to further issues with geneseed that has limited their resources.

As a result Shrike has assembled a special company under his direct command comprising of several specialist units from the 10 companies of the chapter. Deployment varies depending on the situation, could just be a small taskforce for covert ops, or combination of units for more complex missions. They are not always comprised of the elite of the Raven Guard, but are most certainly the optimum combination for the objectives at hand.

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.

@setsneedtofeed
The Court of the Mortive Expanse arose when a Knight world was isolate upon the opening of the Great Rift. Warp-time-shenanigans resulted in them defending themselves alone for a century or more. When contact was reestablished with the Imperium, instead of sending the requested aid, the Dark Angels chapter arrived demanding supplies and when they didn't immediately comply they were declared renegades. Already strained to the breaking point, the Court finally succumbed to Chaos.

My Sa'cea Tau were always supposed to be night fighters. Sa'cea is the closest thing to a "hive world" that the Tau Empire has, and in one paragraph in the third ed codex it mentioned that they were skilled night fighters. But the grey armour Sa'cea wear is very close to Tau flesh, and I wanted helmet less Tau to stand out, so I figured out what Tau, Kroot, and Vespid would look like with severe vitamin D deficiency (and this is back when Tau had cyan, cobalt-based blood) so my Fire Warriors are an unhealthy grey-green, the Kroot are a really dark green-brown, and my Vespid are a calcinated beige/white. It's explained that they spend too much time in the training grounds, and too little under mandated-use UV lamps. Also, bright orange and white markings aren't the best for night fighting, so the Cadre uses a deep red and grey instead. But the Ethereal attached didn't know this before deploying with them, so is covered in garish orange and has a healthy skin tone, which actually helps them blend visually with the army when it's on the board, but still stand out.