Hey there!
Thanks for the input I appreciate it! They are a gun metal grey (leadbelcher), and I used Averland sunset for the yellow

. I am leaning towards Iron Hands, so good guess. I think it's a close one between Ultramarines and Iron hands.
Some of the lore I've created thus far:
They're homeworld is called Zesotera, which means 2nd earth or the extension off earth (I kinda made that meaning up). It's a desert world, and it is somewhat populated. The world is rich in resources vital to the Imperium, and Mars especially.
The main occupation on the planet is mining, and many of the people there are consistently mining the resources of the planet. However, there is a constant threat that the cities have to deal with, especially the tribes and smaller cities on the out-skirts of larger hive cities. The people are in constant fear of beings they call "Rift Walkers". They are like a shade creature, who can manifest in things or change shape. They are very tough to kill.
These beings are a new threat, as the cities there have been thriving for a millennia. Ever since the great rift appeared in the galaxy, did these beings begin to manifest. The Rift Walker threat had got so bad that it began to affect the mining of the precious metals. The space marines were finally called to investigate. Zesotera is also very close to the rift, sitting in between The Rock and Fenris. (Whatever Chapter I decide went to investigate will be the ancestors of my chapter)
The space marines, lets just say Ultramarines - saw how hard it was to kill the shades themselves. Reminding them much like fighting the demons of chaos. But this was something new and just as sinister. There were a few miners/ citizens that had claimed killing a shade. The Ultramarines were extremely impressed with this - that's when the Chapter was formed. The Imperium began to recruit, going out on hunts for shades with citizens who were brave enough for the challenge. If the person managed to kill or help kill a shade, he passed his first initiation.
One man in particular managed to kill 2 shades on his first "hunt". Then preceded to lead many more successful hunts without support from the Ultramarines (They did initially go with him and help train him to be better in the beginning). His name was Lucian (still figuring out last name) and he began to be praised by the citizens. After being continuously recognized, One of the more important characters from
40k lore basically made Lucian the Chapter master. With the help of the Ultramarines at first, recruiting and preparing the worthy citizens for the changes coming.
The chapter is extremely new and only consists of primaris marines. They consist primarily of phobos units, as being hunters, they stick with their tradition of being extremely mobile and trekking for long distances. Shades are near impossible to sneak up on, they usually find you first. So stealth and agility are extremely viable.
Thus the Rift Hunters were founded. Named in honor of their dilemma with the Rift Walkers, but now seeking greater goals: Bringing the lost worlds of humanity back together after the great Rift catastrophe.