Vermilion
Scarlet Red
noun
The kanji and the word are exactly the same. That means they share meanings as well.
朱 refers to vermilion, a bright red color with a slight orange tint, and can also be translated as scarlet red. It's common in Japanese culture — you'll often see it on shrine gates and lacquerware, for example.
The reading can be either the on'yomi or kun'yomi reading, so you have a choice there. Since you learned the on'yomi しゅ with the kanji already, let's learn the あけ reading now.
あけ sounds like the beginning of 開ける (to open). So imagine you 開ける (あけ) a person up, to see what color they are inside. Turns out it's vermilion! Uh… you shouldn't open people up like that.
朱 is mostly read しゅ, but it can also be read あけ in certain phrases, like 朱に染まる, which means "to be covered in blood" (literally, "to be dyed in vermilion").
この布を朱に染めたいんです。
I want to dye this cloth vermilion.