I was thinking about writing a Common App essay with the topic of your choice. I wanted to write about how nail polish has taught me to work better with others.
When I read your description as to what you want to bring up as an idea, I feel you mostly want to talk about uniqueness of every individual.
Again,
I would then discuss how just like some polishes are thin and need multiple coats just like you need multiple interactions with people to see their true personality.
sounds like you need to reflect personalities through nail polish.
In general I feel your idea is to reflect human behavior and personality through a bottle of nail polish and talk about your realizations through what you experience with painting your nails.... So, how about a topic like;
"Seeing life through my nail polish bottle"
First have a sketch as to what are the human traits that you try to match with nail polish and then how you interpret them as your learnings that helped shape your perception about the world.