High temp caliper paint in black or silver
Any other colour will look crap with the Ruebens Red...
exactly right about the colour, although you could also match them to the wheel colour.
High temp paint is unnecessary. Any metal paint will be perfectly satisfactory IF, and only if, you prepare the surface properly. That's also required for high temp paint, so it's no extra work.
Like this:
Remove the caliper complete with its carrier.
Scrape off any grime and loose rust.
Split the caliper from the carrier, and loosen but don't remove the bleed nipple.
Push the piston into the caliper.
Thoroughly clean both parts with solvent cleaner and a stiff brush in a small bowl - cheap nail brush and/or toothbrush.
Sand the
entire surface with 240grit paper
Remove the piston dust seal, and mask the piston, bleed nipple and hose spot faces, and any machined mounting faces. Press the tape down firmly, and finish the edges with a fine file, filing away from the masked surface.
Hang both parts with stiff wire so that you can reach all round them.
Apply a light coat of primer; aerosol is easiest.
When that's gone off(give it a couple of hours), apply enough light coats of colour to give an even finish without runs. Do the same with the clear coat if necessary - silver will need it, black won't. Satin finish clear or colour will avoid an ugly 'cheap christmass decoration' effect.
Leave overnight before unmasking and reassembling.
Of course, such a small amount of the caliper is visible through the wheel that you could just remove the wheel, wipe the visible parts with acetone/thinners/brake cleaner and spray them with whatever colour you've decided on. A lot less work, but it won't look much different