Chuuya is crimson, the color of blood and destruction. Dazai runs his hand through his sleeping partner’s red hair, when he can’t protest (likely with a fist) nor think it means something it doesn’t.
It doesn’t mean friendship, but it is partnership. They share the blood, but not the wine.
Dazai is fascinated by the conundrum of Chuuya’s humanity versus his inhumanity, his equal passion for life and bloody violence. It’s a reason to keep living, to keep looking. Chuuya straddles the line between life and death more closely than even Dazai.
It’s almost odd that he never realizes it.