With their relevant product features and varying line widths, the following paint markers are recommended for marking on rubber: 

• edding 750 paint marker
• edding 751 paint marker
• edding 780 paint marker

It's actually quite difficult to write on rubberlike materials, as any plasticisers present can prevent the ink from sinking in. A very flexible surface can also lead to gaps in the marking. Due to the limitations of writing on rubber, it’s a good idea to test for suitability on an inconspicuous area first.