OK, my primary reason for wanting to do this was that the colours of the kerbs didn't really fit in with my tarmac rally track theme. The problem was the white/red R2 and half-straight kerbs, mix of black and white straight and R3 ones. TBH, I wouldn't have been happy with that mix'n'match look on a race track either.
The solution I came up with was quite simple. Halfords do a "plastic bumper paint" in aerosols. I rubbed the red off the red/white kerbs with Scotchbrite then degreased the whole lot (40-odd items altogether) then applied a coat of the paint. Simples!
It worked on beige borders as well.
The paint comes in black, dark grey and light grey. So if anyone wanted, they could always do the kerbs in a slightly different colour.
If anyone wants to see a pic or two, I'll see what I can do.
I might try some on some spare track as well, if I ever get 'round to it.