Mizu no Iro.

Heaven is a city where life and death meet. Lithia, a boy with amnesia, lives there with Setsu, a hitman covered in wounds. He always dresses Lithia in red, is very kind and laughs as if he is about to burst into tears at any moment, which makes Lithia very uncomfortable. But one day, their usual routine changes completely when Romanée, a man who seems to know Setsu's past, appears...

(Source: Taiyo Tosho, translated)