Mizu no Iro.

Paradise is a city where life mix with death. Lithia, a boy suffering from amnesia, lives there with Setsu, an hitman covered with scars. He is nice and always dresses Lithia in red, and laughes as if he was going to break down in tears just after, which makes Lithia very uneasy. But one day, their daily routine is interrupted by Romanée, a man who seems to know far too well Setsu's past....

(Source: Hana, translated)