染まる: to be dyed

夕日ゆうひで空そらが赤あかく染そまった。

The sky was dyed red by the setting sun.

スサノオが、十拳剣とつかのつるぎをぬいてヤマタノオロチを切きると、川かわの水みずは赤あかく染そまったといいます。

It is said that when Susanoo drew his sword, the Totsuka-no-tsurugi, and cut down Yamata no Orochi, the river water turned red.