My old RRC cap was blue and remained ok while I owned it for 100,000mls same with the rotor arm, my disco cap is now black but still original as the rotor only the plugs and leads have been changed.
I did purchase a Genuine Part cap years ago but it's still in the box on the shelf, very expensive for what it is.
A mate with his 3.9RRC fitted a cheepy dizzi cap around £8 and didn't have any problems with it, I suspect there are some aftermarket caps that are better than others.