Ok people....I dont know if this has ever happened to you!?
My son got an electric guitar for his birthday....now he wants to pretend to play in his own rock band....He cant really play....just makes a noice!
So he puts all my silver chains around his neck and wants me to help him with black eyeliner, like Roderick in Dairy of a Wimpy kid books!
I dont use black eyeliner.....what would you do....play along and buy him some black eyeliner or what!?
Eyeliners come cheap, so yes, I would buy one if I were you. Our rule with toys was always that for birthday and Christmas kids could get some more expensive toys they wanted and then they also got some toys we considered basic needs for them. Other than that they could get cheap ones if they really needed something to the play they really enjoyed. Or they could negotiate buying something they really wanted by paying part of it themselves (usually through doing extra chores etc.)
Your son has nice imaginative play going on and giving him that eyeliner helps making it even nicer. Of course you could also ask him earn this extra toy (as I said, eyeliners come cheap, it wouldn't need to be a big chore to earn it and that would be a good teaching moment also) by doing something extra.
And hey, at least he has understood the right way to start his rock career. Style is everything, learning to play comes secondary or even after that
If you don't believe you can ask from all those boys that went middle school with me and started their first bands then. Or from those who were in middle school together with my sons. And believe or not, one 'style first' boy from my middle school in fact ended up being a rock start (locally.) (There is also one other, but he had boring parents who had put him to music school when he was 6, so he knew how to play before he started to consider style.)
It's also bonus you will have some adorable pictures later. And blackmailing material when he is in his teens
(I have pictures of my sons in pink princess fantasy dresses, with tiaras, from their younger years. Even now I only have to imply of showing old family photos to their girlfriends/friends to get them moving
)