Both forms are correct, but they do different jobs, which is why the spelling question comes up so often.
The noun: makeup
Merriam-Webster gives makeup as one word for the cosmetics, such as lipstick, mascara and eye shadow, used to colour and beautify the face. The plural is makeups or make-ups. It lists the hyphenated make-up as a less common variant, so either is accepted, though the one-word form is the main entry.
The verb: make up
When you mean the action, such as to make up a story, to make up after an argument or to make up the difference, write two words. Its forms are made up, making up and makes up. A simple test is whether you could swap in a different verb: if so, it is the two-word verb.
Which should you use?
For beauty content, one word is the safe choice. Whichever you pick, stay consistent within a piece of writing.
Sources: Merriam-Webster: makeup.
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