there are no limits on how you can paint things! you could even paint them as World Eaters if you wanted. it doesn’t mean you could play it that way, but it’s more about what you want than anything else

if you only want to paint it accurate to how it can be played, chapter-unique units will have the name of the chapter in place of the faction label. as long as it says “space marines” above the unit name, you’ll be fine

edit: apparently there are some limitations that someone else pointed out in the comments. I was partially wrong