Any spray paint that's desinged to be used with plastic should work. The paint itself has to be flexible enough to not split / crack when the fenders flex under normal use.
I used, Krylon Fusion for Plastic, and know it works very well. Also not expensive, $6 - $7 a can. I easily did two fenders with one can, and have between 1/3 to 1/2 of the can left.
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