duplicolor sells a paint kit now for brake calipers you might look that up.
You can to go to forums.rennlist.com and do a search and find calipers of all colors on Porsches. Mine are yellow on my GT3. i would be interested in seeing how this works out for you. I would like to do mine as well.

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2007 335i Alpine White(moms)
2006 X5 4.8iS Sapphire black(Mine)
2007 Porsche GT sliver GT3