[MrFlippant]

Another option is rubberized spray paint, aka "plasti-dip" and similar products. It sticks well without laborious prep, and can peel off if needed. Spur gears can damage it, but I expect that is the case with the other options. In the past, black was the only option, but as competition increases, so have color options, so it's worth looking at other brands to see what they offer as far as colors... if you don't want black.

Also, on another note... one of the reasons a lot of people prefer the black borders is because the kerbs are flat rather than bumped as the red/white ones are. This reduces the disruption of cars that slide out over the kerb. If you are wanting the flat borders, there are models for 3d printed kerbs that fit into Scalextric Sport borders. You still need the borders, but can pop out the bumped ones and pop in the flat ones. Here's a link to the freely shared models, but I can also be contacted for prints of the models @ 2 for $1 if you need someone to print them. Any color. :)

https://www.printables.com/model/284674