I am not a chemist, but if I am not mistaken, any PVC that you burn gives off toxic fumes. That sounded like an excuse. Most likely there just isn't a very big demand for gray PVC pipe.
I am not fully understanding your diagram. It's hard to figure out where your plumbing will run until you get your 4Way. You will have to deal with the fact that there are outlets all around it. I would suggest using double male close nipples and threaded unions to connect to the 4Way. That was the easiest combination that I found that allowed easy removal of the 4Way and used the least amount of space.
Use 45s off of the 4Way to square up the plumbing, like this:
If I were you, I wouldn't paint the PVC. Not because you can't, but because it would be a real mess trying to do that after everything is assembled. If you paint it before, you will have a mess when you try to glue everything up. Probably best stick to white. You won't be able to see your plumbing from your viewing area anyway. In my case, you can see a little behind the tank and the gray is just less noticeable.