|Founding||First Founding (later retconned)|
|Specialty||Being forgotten, dressing up as space Aztecs, being cryptically referenced again.|
|Colours||All the colors of the Rainbow (No durr...), but mainly Blue|
Back when Warhammer 40,000 was still called Rogue Trader, all of the First Founding Space Marine Chapters were known, and the Rainbow Warriors were one of them. Nothing else was known of them, except for the fact that the Sisters of Battle curb-stomped them at one point. Yeah, Games Workshop didn't think too highly of these guys, and in a Third Edition White Dwarf article, they were declared to be of a "later founding." They also hold the title of silliest space marine chapter name of all time.
In the Fifth Edition Space Marines Codex, the planet "Prism" is designated as the homeworld of "<<record deleted>>," a chapter which bears the Rainbow Warriors' insignia. Prism has been found! Get the 6th edition and look in the Northwestern border of the Vidar Sector located in the Imperius Dominatus.
Somewhere along the way, people thought it would be cool to envision the Rainbow Warriors as Aztec Marines.
It is noted that the image for Sisters of Battle in the Rogue Trader rulebook has a Sister shoot down what appears to be a Rainbow Warrior. This combined with the record deleted statement above suggest they were filthy heretics who got purged.
There's also a basically-the-Rainbow-Warriors chapter called the Death Strike. Who hate the Ultramarines for helping to excommunicate their brother chapter. HMM.
They're still around in the 41st Millennium if the 6th Edition Codex is anything to go by. They, the Relictors and the Fire Lords fought a bloody powerful Demonic incursion, but ended up losing a fair chunk of their chapter - including their Chapter Master.
There's a fair chance that they were named after the Greenpeace ship, the Rainbow Warrior, which was sunk by French secret agents one year before the release of Rogue Trader.