i use whatever on slaps highlighters sharpies white out make one and go put it somewhere it wont get removed and go back later and check how it did
Go to markal'so wbe site and fill out a sample request form with a bogus company name. They have a fantastic semi fine tip marker with weatherproof UV resistant ink, paint sticks, and paint markers. It's free.
Maybe for indoors? Crayon is a bold medium for this kind of "art" so it's probably a big hit or miss.
People tag with grease pencils. I never have. Despite being water resistant they do wear out and fade over time.