朱
シュ, あけ, あ, あか, あき, す

vermilion, cinnabar, scarlet, red, bloody

JLPT1 G7 S6 F1788

The relevant seal inscription form is 木# tree with a horizontal line through the middle, suggesting a stump → *red*; *vermilion* (← color of a freshly cut stump).

[Full Kanji Etymology List]