Following on from the lazy quick fix above which will leave you with black hands
I purchased some leather acrylic paint from Angelus after watching numerous YouTube videos of how people use it to repair trainers and handbags.
Cost about a fiver each and £2.50 p&p.
They recommend a deglazer first, but it's just acetone which is basically nail polish remover!
My Zeds side bolster has started to show wear after 13 years of being my daily despite climbing
2 light coats of the Matt finisher/sealer applied and dried and the finish now blends into the surrounding perfectly.
The Flat Black from Angelus is a perfect match for the zed black leather.
The amount of gloss/sheen from the top coat sealer can be dulled further with the addition of Angelus Duller mixed in with the Matt Acrylic Finisher.
If my seats were new and more matt I think the Duller would be required but on 13 yrs old seats the Matt Acrylic Finisher is just right.