Hello. I have some body parts I want to colour code, bumper overiders and headlight rings. What's the method for doing this please?

If they are Aftermarket parts with " dry climate" chrome, just leave them out in the rain, and it'll rust-off
If they are genuine VW proper Chrome, don't try to remove it, but 'key' the old chrome surface with abrasive, and use a good primer before painting.
We did a Europa bumper in black , couldn't get the original black anodised parts, but plain steel bumpers for painting, were available, (only from VW, late model 1200 Basic Beetles had silver painted bumpers), but they cost more than after market chromed, so that's what we got.
Gave it a good rub down with emery, and used an etch primer (not sure if that was really necessary), then a few satin black top coats. That was about 10 years ago, just starting to show it's age at the edges.

Key the chrome, thin coat of etch primer, once dry lightly sand back, another thin coat of etch primer, let dry then normal primer then paint.

Did it some years back for my GL bug and the bumpers are still good today. This was aftermarket 'better quality' bumpers.

Etch primer will have no effect on chrome as it is almost chemically inert (which is why it stays so shiny). Abrasion is the key, literally, and very few abrasives are hard enough to have much effect. Be prepared to use a lot of conventional abrasives, as the hardness will dull them very quickly. Any aluminium oxide-based paper will not have much effect; silicon carbide or tungsten carbide will be better, however only diamond will be truly effective. Once it is keyed it matters little which primer type is used, but I have found that epoxy seems to last best. For a proper finish it would be advisable to have a platers strip the bumpers back to bare steel.