That is because it has to be uniform. The plates were developed over decades of testing in order to be able to reliably screen large numbers of people for color vision problems
. Once identified, there are more specific tests that can be done.
The candidate, in the situation you described, does not have any protection from this sort of action by the employer.
That being said, if a person can not pass the color vision plates they would not be able to function in a job that requires excellent color discrimination, such as a Navy fighter pilot or a telephone wire splicer...
Does this make sense to you?
It's safe for you to press the positive feedback button now if you so desire. And, never fear, even after you press that button I don't go up in a puff of smoke -- I'll still be right here to continue helping you, but, as I do work for tips, I want to make sure you are happy before rating me.
One other thing: I notice that this is the third question you have asked on this web site. As you might not know, if you do not press the positive feedback button the expert is not compensated for assisting you. And, since this the lowest priced question you have asked, I would like to suggest that you mash the positive feedback (excellent is most fun to push) button at this point.