i have a few ideas so let's go down the rabbit hole of what my 'tism fixation has brought upon us today.

i think he started to steal it from robin, but when he got the car, he just started jacking it from CVS / the mall. i just know, he WAS that late 90s/early 00s emo in hot topic glaring at other people in the store while he stole makeup because of their band shirts or aesthetic or something fr. (then, he gets hired there [in my personal headcanon]).

i don't know if anyone's had the hot topic makeup, especially back in The Day, but it was actually decent for it being Hot Topic makeup. The eyeshadows are chalky sometimes and stubborn to get an even, vibrant layer from, but they excelled with the eyeliners and nail polish. (i also used to work there.)

usually, he gets a pencil eyeliner. it's simple to use with no makeup experience or knowledge (sam's brain like a caveman: pencil. pencil must be sharpened when dull. then draw on eye. simple.) but the first time he did it and saw the sharp, messy lines, he definitely hated it. then he smudged it out and voila, his identity has been found.

at least, the 2001 older emo son who loves manson identity (i make him better because really? manson? but also it was 2001 and manson was extremely popular then, but it didn't age well.)

sam started out with a hot topic pencil eyeliner and a dream. and a whole lotta weed. and eventually, he found what he did like. plus, he enjoyed whatever reaction it caused to George, Peter, Robin, etc., because he honestly did things just to piss them off. (i.e. calling Bob about the toilet near the kitchen.)

however, the more the makeup industry expands, I, personally headcanon, that he would be curious even when he goes through that period of not wearing makeup (again, mostly because of the "feeling lighter, looking lighter" trope that LAAH uses for his character that i REBUKE because you can be happy and be emo goddammit. i know it's for storytelling and visuals but STILL.) he'd see the new products and things and eventually, he would buy a few to test out and then sam's journey into identity and self discovery begins (he's definitely a little fruity, definitely autistic, definitely emo but he's definitely not as angry anymore, but his heart still hurts from that summer).

he also gets some damn therapy.

anyways. he would definitely be picky with makeup ☝️ ONLY because of sensory with the autism. he enjoys eyeliner over shadow because of staying capability and blending, but sometimes you can only jack an eyeshadow from your local hot topic. originally, he did steal from robin, but she only has one eyeliner and its dark brown and to this emo fucker, it wasn't dark enough to satiate his emo needs.