We have an inspector's position open. The job duties are to perform detailed inspection on parts we manufacture. Our customer requires that their parts be inspected by someone who is not colorblind. Can we ask, in the initial screening process, if they are colorblind without violating any laws?
Edit: I should add that if it wasn't for the customer's requirement, a colorblind person can perform all other job duties. They would pass our practical test.
If being able to distinguish colors is a bfoq - Bona Fide Occupational Qualification - then you may ask about it or test it and disqualify people from hiring based on the results.
In fact, a Jury found that color vision is a BFOQ for police but demanded accommodation and in that case the Appeals court denied accommodation is necessary. Other courts found that at times color blindness doesn't count under the ADA at all. Some federal regulations even demand specific ability to have some 3-color vision, as you need to distinguish Red-Green-Amber.
Remember Different color blindnesses see differently, and the following picture illustrates this at a traffic light. Deuteranomaly alters the color picture different than Protanopia, which is different from tritanopia, and then there's the unpictured achromatopsia, which is totally Black-White, among many many other types of color-blindness known to man:
Can someone with any of the three pictured color visions distinguish the traffic light colors but or the position in the traffic light? That is the question that the federal regulation for color vision above demands, making it a requirement for truck driver licenses.
For example, a job that relies on distinguishing colors would be sorting or quality control based on the paint of a product. Take for example this sample from the AOA (American Optometry Association) for testing color blindness:
Assume the products come only in the colors of the second row, and only those. If you can't distinguish between the ca. 6 greens, yellow, and about 7 variants of red, then they could not have the qualification, no matter what other qualifications you might have, you could not fulfill the required qualifications for the job of evaluating the color of these items.
On the other hand, a colorblind person can be especially valuable when you deliberately look for color combinations that are still well distinguishable even to people with color distinguishing impairments - and then being normal does not qualify the Bona fide occupational qualification.
Can I ask a potential employee if they are colorblind?
Yes. Sutton v. United Air Lines, Inc., 527 U.S. 471, 490-491 (1999) held that
[A]n employer is free to decide that physical characteristics or medical conditions that do not rise to the level of an impairment—such as one's height, build, or singing voice—are preferable to others, just as it is free to decide that some limiting, but not substantially limiting, impairments make individuals less than ideally suited for a job.
Id. at 493 cites 29 CFR § 1630.2(j)(3)(i) to points out that, because the position at issue is a particular profession, the disability does not constitute a "substantially limiting impairment". Indeed, the candidate's condition of colorblind would not disqualify him for other jobs.
Accordingly, the approach you outline in this comment is unnecessary. In certain contexts it can even backfire because a jury might find that this was an attempt to elude legislative intent.
I think we're looking at the wrong end of this question. There seems to be a dispute between you and this customer on whether colour perception is required for the inspector's role. That is the point to resolve.
If you believe that colour perception is a requirement, it would be legitimate to ask the question, and use that to select who to appoint. Discrimination where someone would be unable to carry out the duties of the job would not be illegal - unless there were reasonable accommodations you could have made.
But this only works where you've made that decision and are willing to stand by it. If a rejected applicant was to allege discrimination on the basis of a protected class, answering "because the customer said so" will not help you with a compelling legal case.
