Celebrants
The Celebrants are a loyalist Space Marine chapter of unknown founding (sometime in the 36th millennium) and origin but who have one of most original colour schemes yet seen in a loyalist chapter, going from red at the helmet down to yellow at the feet and all the shades inbetween. GW upped it's game on the artistry side for the colour schemes.
Besides playing a part in fighting in the third war for Armageddon (who didn't fight on that damn planet???), the only other mention of the Celebrants as a chapter was during an Alpha Legion plot to destroy another loyalist chapter, the Crimson Consuls, by arranging for the Celebrants to be moved to guard a cathedral world from the Tyranids while the Consuls took their place. Cue attacks on all elements of the Consuls and their utter destruction (just as planned). Their geneseed was then taken by the Alpha Legion and presented to the Despoiler so he could build up the Black Legion in time for the 13th Black Crusade. Hopefully the Celebrants were not aware of how they were involved in the Alpha Legion's dickery.