try cheapo hair spray in a pump bottle. Worked great for getting sharpie out of the headliner and cloth seats in the mini-van when my eldest decided to try out his writing skills.

And then you can comb the saddle any way you want.

When my then 2 year old did sharpie on light grey leather car seats I got desparate and used acetone...it worked. Car upholstery shops have rattle can leather finish....don't know if they have bright white.......

other options......
a) big bossman's suggestion of having it covered by a cobbler or probably better yet an auto or motorcycle seat upholsterer.

b) Get thin white leather and 3m spray adhesive and do it your self.

Maybe you can buy a white vinyl headliner from an autoparts store and recover the saddle.

hot rubbing alcohol, but if it's leather you will need to restore the grease back into the saddle afterwards with mink oil or proofide or similar.

edit: oh: it's vinyl, so go for it.