They're alloy, with a cast iron/steel base right?
If so, the best way to "restore" them (i.e. if Holden was making a cut-away demonstration car in their museum etc. with no funky colours) would be hydro/vapour blast the alloy, and zinc plate the cast iron/steel (you can zinc in silver - they call it blue - or gold, or low gloss black, but the black isn't as hardy a finish as the first two).
Vapour blasting the alloy will close the pores up a little to keep them looking nicer and cleaner than the fresh cast finish (particularly after they've had 20 years of use). This is what cars like Ferraris and Porsches etc. get all of their cast alloy driveline parts, steering arms etc. done with during restos.
On the alluminium bits, the only "plating" you're going to do would be anodising. You could also polish them if you wanted that look but it won't be an awfully low maintenance finish.
Paint/powercoating will do a better job of closing up the pores.
I don't imagine many people want to show off a sliding caliper design by painting it a bright/light colour. I would just be media blasting them and painting them with black or gunmetal caliper paint, personally. It seems to have reasonable longevity and won't look like it's dirty very quickly.
Vapour/hydro blasting before:
After:
Edited by Heath, 08 February 2021 - 01:59 PM.