Hi - thought I'd share some photos of an excellent repair carried out by Peter Crook of Carneil Pens in Kirkcaldy, Scotland.
I had been after a blue marble Parker Duofold (1990s) for some time, and got one from eBay. When it arrived, I discovered the lip was cracked, which apparently happens quite often due to overzealous cap tightening.
The original lip is blue marble, like the rest of the cap except the top. Peter informed me that only black lips are available, so I went with that. I'm actually very happy with the result, I think it looks as good as the original. The black lip matches the top of the cap, and mirrors the black ends of the pen itself.
I've attached some photos of the repair. Many thanks to Carneil pens for a great job.