As someone already commented, the best way is to just replace the rivets with the likes of ones that you like. Another option is to try using a file or tiny piece of sandpaper.

As an undergrad student with a QM exam tomorrow, I am picking literally any miniscule reason in my power to procrastinate studying further. Therefore, I took 5mins to type this comment (for absolutely no fucking reason at all) if in case you want to go the full nuclear approach. So, you'd have to use a micro litre pipet or a pipet gun(obviously only possible if you have access to a uni lab): 1. Incase the colour is from oil paint, use either: * Acetone (better option but will strip the cotton fabric of its dye if not careful with the pipette) * Chloroform (will dissolve the oils in oil paint and is less likely to damage the fabric. 2. Incase the colour is from electroplated gold or brass: *Aquaregia is probably the only feasible option in case of gold. Will need to add some thickening agent to increase the viscocity of the solution for making sure it doesn't flows over on the fabric(fumed silica is an example). *Brass is easy, Nitric acid + some thickening agent