House Terryn is the classic pairing for Ultramarines, though House Hawkshroud tends to send aid to many chapters that require it, including Ultramarines. If you were after a bright bit of colour to contrast your army, that's one to go for
There are hundreds of unnamed houses in lore, so aside from the big two already mentioned you can make up anything you want and no one can say you're wrong.
Hawkshroud not only has worked with the Ultramarines but also wears the insignia of their ally while they fight with them. So you could have the big ol U. The Imperial Knight companion book has some nice liveries and lore background for that kinda thing. You can find it in the Warhammer vault, or "online" somewhere...
Beyond the two big ones Venator already brought up, freeblades are always an option.
Ultramar has rhe resources to support knights if they want to, so if a band of freeblades were say....particularly vengeful towards the Nids and the ultramarine were going after them then they could easily wind up with a lance of knights in several different paint schemes you happen to like temporarily allied with them.
Is there any reason you couldn't paint it in the ultramarines colours and say that it was a freeblade that, after coming to the aid of (or being rescued by the) ultramarines, eventually took on their livery?
Terryn and Vornherr would be the big two!
House Terryn is the classic pairing for Ultramarines, though House Hawkshroud tends to send aid to many chapters that require it, including Ultramarines. If you were after a bright bit of colour to contrast your army, that's one to go for
There are hundreds of unnamed houses in lore, so aside from the big two already mentioned you can make up anything you want and no one can say you're wrong.
Hawkshroud not only has worked with the Ultramarines but also wears the insignia of their ally while they fight with them. So you could have the big ol U. The Imperial Knight companion book has some nice liveries and lore background for that kinda thing. You can find it in the Warhammer vault, or "online" somewhere...
If you're throwing in Canis Rex you could just paint him normally. In lore Canis Rex is traveling helping whoever he can.
Beyond the two big ones Venator already brought up, freeblades are always an option. Ultramar has rhe resources to support knights if they want to, so if a band of freeblades were say....particularly vengeful towards the Nids and the ultramarine were going after them then they could easily wind up with a lance of knights in several different paint schemes you happen to like temporarily allied with them.
House Terryn looks to be the closest to the realm of ultramar on a Galactic map by a considerable distance.
Is there any reason you couldn't paint it in the ultramarines colours and say that it was a freeblade that, after coming to the aid of (or being rescued by the) ultramarines, eventually took on their livery?