Farther than stars said:
Yeah, best if this stays a hypothetical, huh? It's interesting though, because a lot of animals only see certain colours, primates actually having a rather wide spectrum which they can see. And I also wonder how some seriously colour blind humans operate with those issues you mentioned.
Well, they can see those colours as shades of grey, not as invisible. However, there are practical uses for this...hunters wearing blaze orange so they are highly visible to each other, but not to their prey, which sees them as drably coloured, for example. Tiger camouflage works that way as well, except tigers can't see each other as brightly coloured.
You never, ever, ever see this in science fiction. Nobody makes camouflage patterns based on what colours the aliens can or can't see, nobody has tac-lights in colours the aliens can't see, etc. You could get away with crazy garish uniforms from the 1800s if you said it didn't look like that to the aliens. I'd love to see it explained that the reason British soldiers wore those bright red shirts was because the crab people saw them as drab grey.
As for human colour blindness, it's an issue, yeah. Alot of things are colour coded nowdays...most obviously green for go and red for stop. One of the most common colour blindness renders you unable to differentiate between red and green.