Can you become a crop dusting pilot, if you're color blind?

Depends: Color-blind pilots are typically issued a restriction reading, "not valid for night flight or by color signal control." this mitigates the concern that color-blind pilots may not be able to identify those colors required for the performance of safe airman duties by preventing situations that are considered potentially unsafe. More info: wikipedia pilot certification.

Probably: The faa license requires 20/20 uncorrected or corrected in each eye for first and second class for distant vision. For near vision, the requirement is 20/40 uncorrected or corrected. If you are 50 or over, there is the additional requirement of 20/40 for intermediate (panel distance) uncorrected or correct. You can get a color blind waiver.