The Rainbow Warriors

Here's what we know:

Heritage:

Just another generic Ultrasmurf successor chapter that (presumably) adheres to the Codex Astartes (yawn). 

Colors:

Copyright of GW

Dark Blue Armor with a rainbow stripe on their helm, unit markings as per the Smurf standard. The official chapter icon is a monochromatic white rainbow (huh?) often misidentified on 40k Wikis (just one example) as well as by myself (initially, till Neverness set me straight) as a 'wing' with a lightning bolt attached. 

Homeworld: Prism

Questionably Canon Fluff:

They fought in the 'Blood Star Campaign' alongside the Relictors and Fire Lords. All 3 chapters suffered heavy casualties which included the loss of all 3 chapter masters. The Rainbow Warriors are listed in many places online (forums, etc.) to number only about a 100 hundred marines or so after coming into conflict with the Inquisition. Which branch isn't mentioned, but given that the 1st canon appearance of a Rainbow Warrior depicts one getting shot by an SoB, its safe to assume that it was the Ordo full of Heretics, and their SoB lackeys. I have yet to uncover where that bit of info actually comes from, but its fairly widespread (regardless of canonicity). 

...and so the treachery begins...

Starting with the 6th ed SM codex, the Rainbow Warriors' homeworld is listed as 'RECORDS DELETED' which is the treatment usually reserved for renegades and traitors, not simply a chapter that fought to the last in the Imperium's name. No doubt the Ordo full of Heretics is responsible for this as well…

Notable features:

Of course, its their dumb sounding name. Well, unless of course you were either a Jeff Gordon fan, or are an 'eco warrior' who may or may not still be angry about the sinking of the Greenpeace ship of the same name. 

My take on the Rainbow Warriors:

After continuous and wrongful persecution by the Ordo Hereticus and its nigh unlimited resources, the Rainbow Warriors suffering near 90% casualties were broken as a chapter. Their command structure ceased to function and/or was wiped out and the surviving Rainbow Warriors scattered across the stars in a desperate bid for survival. 

Whilst a few Rainbow Warriors may have turned renegade or worse, others have remained steadfastly loyal to the Imperium which had betrayed them. Regardless, the majority fall somewhere in between. With the goal of survival surpassing all else, the Rainbow Warriors have had to rely upon piracy to resupply themselves. An unfortunate side effect of these raids is that their numbers continue to dwindle...

*****

Those two scouts have been partially repainted more times than I can count! Their armor is a mix of Rainbow Warriors blue and black armor and bolters which could have been stolen from a whole list of chapters.

As such, my Rainbow Warrior Kill Team numbers just 10 marines. Six battle brothers and four scouts. I'll be using the Black Templars Crusader squad for organizational purposes. The BT chapter tactics btw, will rarely ever come into play in a KT game, and thus don't really matter. Lacking the needed facilities to complete their implantations, their scouts will likely never achieve full space marine status (which is fitting since I still use the 6th ed codex, and thus my WS/BS:3 scouts still suck). 

Note the scavenged Dark Angels leg armor (and possibly the bolter as well), also that particular chainsword design hasn't been in use since the Horus Heresy!

These piratical raids have resulted in patchwork-like appearance of the Rainbow Warriors' armor (stealing a page from the old Space Wolves 13th Company). Similarly, their weapons have no standardization of model or coloration. Only their pauldrons and helms are given attention of conformity, as they have the chapter's symbols on them. 

The infernal camera once again
points out so many needed
touch-ups, ugh...

Lightning bolts above a winged bird's skull, adorned with feathers that are rainbow colored for battle brothers' pauldrons, and just lightning bolts for scouts. A 'tactical arrow' on the Battle Brothers' helm denotes which company they were once a part of, whereas individual squad markings have been removed.

Transport for my team is (ironically) a stolen Inquisitorial Rhino, though I'm looking to eventually add a small bike squad for a bit of variety. As you can see above, I have three fully painted thus far. Luckily there won't be that many total marines to work on, as my version of the Rainbow Warriors' chapter icon would no doubt get tiresome to paint in large quantities.