In Canada it is against the Law to discriminate against people based on Vision. However any employer is permitted to ask for a Medical Prior to Employment. Providing the Criteria in the Medical is Bona Fide ( Meaning in Good Faith ) .
They must be able to prove that Color Vision is not required for the Job. If you have Certification that enables you to perform the Trade without a Medical needed . Then to me the employers need for Color Vision is Not Bona Fide as such, and if you were in Canada I would proceed with a Claim through the Canadian Human Rights Commission.

As you are in the USA . I do not know if they have a Human Rights Tribunal there .

Good luck with your Endeavors.

Sam

Just as a Note;
If you are in fact Color Vision Deficient. There appears to be some good News on the Horizon for you and many Millions of other Caucasian Men.

Watch this Video. It is only a few days old. And something certainly worth keeping your Fingers Crossed for.

http://www.youtube.com/watch?v=0IBT-jGja28

Sam

http://www.youtube.com/watch?v=0IBT-jGja28