Honestly you don't need red. Like, you get to add Ulasht, the hate seed to your deck and...maybe Apocalypse Hydra and that's about all I would consider. Hydras should be Simic if anything because you might as well enjoy the full wonders of big mana.
Techniquely utility for getting damage though is in red
OOOOH SO CLOOOOOSE! IF THE HYDRA WAS GRUUL IT WOULD HAVE BEN ARE HYDRA TRIBAL LEGEND WE ALL HAVE BEN WAITING FOR!
(but really green is in a way all you need for hydra tribal since most of them are mono green)
and the blue knight
Sweet it’s etb is brainstorm and a shuffle your library trigger
Techniquely utility for getting damage though is in red
https://www.edhrec.com/tribes/hydras
Also X costed stuff is usually the subtheme
(but really green is in a way all you need for hydra tribal since most of them are mono green)
and the blue knight
Sweet it’s etb is brainstorm and a shuffle your library trigger
Too bad it doesn’t have flash