I agree with ncx, either paint them silver or black. I would reserve colors like red, and yellow for a bbk (brembos, stoptechs, ap racing, etc). Painting the calipers a loud color warrants extra attention to them, and the stock calipers are nothing to show off. I have Brembos on all four corners of my M3 and got them in silver, didn't want something too flashy.

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