Mucus color can vary, but usually the darker it is, the more likely something is in his lungs. I think it also depends on the individual. When I'm well it's usually no darker than a faint peach color and is more often white. Is his saliva thick? Some CFers experience that phenomenon, myself included.
I think if I was in your position, I would tell him what is going on. Tell him the name of the disease and how they test for it. The sweat test really isn't anything but time consuming -- maybe a bit uncomfortable at most. They're likely going to take a significant amount of blood, but it happens. Anway, I think I would tell him that it's just a test, and if it comes back positive he's the same person he was the day before, and everyday before that. Explain to him it's just an outlet for a possible answer to all his strife, and that the advice and treatments that comes with that answer might really make things better for him, if he's willing to accept that answer.
If the results are positive, make sure he knows it's not his fault -- it's a genetic disease, and if he has it, it's simply part of his genetic makeup. It's how he was created -- something he had not the slightest bit of control over -- but it's not a definition of who he is. If he's frustrated at his current health situation, not only could CF treatment make things better for him physically, but it might help his emotional anguish by showing him that his lack of improvement to date hasn't been his fault. CF, when undiagnosed, is an inexplicable detrimental element in day to day life. When it's brought into light, though, things can really change.
If he is diagnosed, it might be a tough pill to swallow -- especially for you as a mom. There is certainly nothing you did wrong (unless you're a smoker, in which case that will need to come to a stop IMMEDIATELY). You're going to have more questions than your brain can contain when you're speaking with the doctor, and the most obvious research tool when more come flooding into your head in the middle of the night is the internet. It has a lot of helpful stuff about CF, but you're going to see facts and figures that will, excuse my language, put you on your ass (that's the only way I can describe it). Be careful about what you absorb as fact. Just like every person is different, everyone with CF is different. He'll be even more different if he has a rarer mutation.
I wouldn't keep him from doing his own research, since it is his body and he deserves to know what it's up to. You could always look things up before him, though, and recommend sites that may not scare him as much. I would give him the same speal about everyone being different before even getting tested, and reinforcing the idea well after the results of the tests come back. Also, I would get him something to take his mind off of it. Maybe buy him a video game or, even better, something that involves more activity (new bike? roller blades? Even a guitar or something to spike his interest in some creative facet so his mind doesn't stagnate on the diagnosis/waiting on results.) It sounds materialistic, but I think it's both realistic and reasonable, especially at 12.
This might be a lot more than you wanted, and I don't want to get your mind reeling about what your future could hold for your family if your son is diagnosed. If things do come down to that, though, at least you'll have an inkling of an idea about the process. Nobody is going to be happy about it, especially once your son hears about the daily therapy (if they demonstrate the vest, it's going to blow your mind, but it does work). If it hasn't sunk in yet, though, try remembering ignorance paired with CF is not a road down which you want to travel.
That is why we are here together on this forum. To share information. And we will be here to give help if you need it.
Try your best to have a good day. Rinse. Repeat.