According to Google, colorblind means this:
- unable to distinguish certain colors, or (rarely in humans) any colors at all.
The problem with this scenario is that 'colorblind' is a huge misnomer (in one sense of the term). Most colorblind individuals can actually see a great deal of color (many can see most of it, in fact). However, they may not see enough of certain colors to be able to pinpoint exactly what a particular one is. Like, they might not see a certain kind of sea green very well, but forest green is plain as day—stuff like that. I can see pretty much every single color in cartoons very easily, but many car colors and clothing colors are quite difficult by comparison. The size of the colored object matters. The distance also matters. The light levels also matter. How dark or light the color is matters. I can see some colors easily in some situations, but I may not see them at all in others.
Anyway, you need to acknowledge that your client really can see colors—unless he's one of the extremely rare people who really can't see any. If you just say he's colorblind, he'll feel like you're picking on him, labeling him, misunderstanding him, etc., even if (and perhaps especially if) you prove it to him with a colorblind test without validating him first—that can actually be traumatic, and I've seen people be in denial about what their test results mean (a relative and a roommate of mine).
You need to recognize that your client's perceptions still have value in the real world even if he's colorblind. You can't just write him off as colorblind and say his color-vision doesn't matter anymore, and not just because of the misnomer: Colorblind people are actually capable of seeing some colors that cannot be perceived easily, if at all, by color-normal people. For instance, if an object is a mixture of red and blue, but doesn't have very much blue in it, someone with red-green colorblindness may notice the blue in it easily, while a color-normal person will probably think there's no way there's any blue in it (unless they're used to adjusting RGB values on websites or something and aren't into inductive logic). So, when they say something looks like a different color, that doesn't mean they're wrong, per se (at least if they're not just guessing what color it is, but they actually see it that way). What they see exists—they're just not seeing everything. It's kind of how white contains a lot of red, blue, green/yellow, but you can't really see any of those colors in it, unless you take some color out of the picture. If yellow is the only color you can see, white will look yellow (because there is yellow in white).
Anyway, the definition of colorblind is also flawed, because there are colors that most humans can't see (but somehow, that doesn't make most humans colorblind, while if you can't see a few rare kinds of red and green, somehow you are colorblind). For instance, most humans can't see UV or infrared, but some people can, and birds can, too. Does that make everyone else colorblind? It's a double standard to say that because you can see these colors, but not those, you're colorblind, when color-normal people can also see colors from some wavelengths, but not others. I don't know anyone who can see colors made via microwaves, but I'm sure they exist.
Anyway, I think you need to make the person feel like you're being fair. You need to acknowledge their capabilities (that they haven't been completely delusional their whole life), and let them know what colorblind really means. (It doesn't mean you can't see any colors—nor does it mean that the person should be treated as if they're inferior. It might also be worth noting that some colorblind people are not affected by camouflage.)
I believe you should also recognize that just because your friend with red-green colorblindness can't see a certain thing, that doesn't mean that your client can't. There's usually a lot of variance between two people with colorblindness.
Also, be careful, because your client might not be colorblind at all. It's possible he was just starting at some bright color that temporarily blinded him to the color at hand. I know if I stare at a certain kind of bright pink (which I can see) for a short while, and then look at other things, the colors look different for a time. There may be other scenarios like that.
Also, realize that colorblind people are sometimes actually already aware that they have trouble with certain colors, but they don't realize that this makes them colorblind. They may think everyone has such troubles, or that it's not significant enough to make them colorblind. However, they can gain a lot more insight into what color is what if they know they're colorblind and admit it (this is my experience, anyway).
Anyway, so for my answer, I'm going to have to say, be sensitive to the situation with the things above in mind. It's not easy to convince someone they're colorblind—but if you're going to do it, at least make sure that everyone concerned knows what 'colorblind' means first, and that it's a misnomer (they usually don't). Don't gloat. Don't remind the person that they're colorblind at the slightest provocation thereafter. Don't talk about how you can see more colors than they can (and do realize that if you haven't been tested, you might be colorblind, too, even if you can see those colors that your client can't). Please do not unnecessarily reveal the person's condition to every Tom, Dick and Harry in their presence, even if they're not in denial about it (but especially if they are).
I know a lot of these things I've said not to do are tempting, and I've done some of them myself (which I shouldn't have); but, I'm not trying to condemn anyone for having done such things. People make mistakes, but we can all move on, learn, and become better.
Treat the client respectfully. Treat the client as an equal. Things might not go so smoothly, but it's not the end of the world. As long as you're respectful, I don't think they'll hate you. They may be in denial about it, or not like/believe what they hear, but if you don't otherwise offend them, they may be more likely to one day be glad you told them—and it probably won't hurt your relationship. However, if you rub it in their face that they're colorblind and act like they can't see any colors, then they're probably not going to want to deal with you so much.
If you don't have a legitimate need to convince someone that they're colorblind, from an interpersonal relations perspective, I agree with others that it's probably best to leave that to the professionals (or the individual themselves).