The purple stain,
the same shape as your legs and tush,
which gradually builds up on a white or light-colored toilet seat is the result of the reaction of the body chemistry of the surface of the skin of some people mixing with the finish of the seat.
I`m not a chemist or a doctor,
so the exact chemical reaction is not something I can address.
But I do know that the only thing that will remove it is straight acetone,
which is available from beauty supply stores.
Don`t leave it on the seat for any length of time.
Just moisten a white paper towel with it and scrub - it will transfer the purple stain from the seat to the paper towel.
Then allow the residual acetone to evaporate and clean the toilet seat with any commercial cleaner made for this purpose.
The acetone will work best if the finish of the seat has not been scratched or worn away.
The stains will return over time unless you go out and buy yourself some new skin.