At my MEPS they do 2 color vision tests. The first one is a Picture-In-Picture test, if they fail that, they take the Falant (spell??) test, if that's passed, then they are considered to have color vision. They are given the tests again in boot camp. And that second test, I think, shows you a number of things, if you see a certain number, you pass. If you only saw the minimum number, then when the test is given again at boot camp, if you see one less, you fail.
I apologize for being so vague.
I know that prior to shipping to boot camp, if an applicant is determined to be color blind at MEPS, then they can go to 2 independent doctors. If the 2 doctors give passing results on a color vision test, MEPS will accept those results. I don't know if that avenue is available once someone has already shipped to boot camp.