Uhhhhhh...You'll want something in the acrylic family. It'll make a difference whether it's indoor or outdoor. In either case, you'll wanna talk to someone at a hardware store and ask them for a paint that'll work on glass.
I know that art acrylics will stick, and the price varies by pigment and brand. (They will be able to be scraped off with a razorblade, though.) And you'd want to put some kind of sealant on top to protect the painting when it's finished.
Pretty much everyone agrees that watercolor is just an inherently tricky medium. It just takes time to practice and figure out how each different pigment behaves.
That's what makes it exciting. ;D
Actually, I think that that very factor is so freeing. When you work digitally, you can redo something over and over and over again until you get it "right". It encourages you to be a perfectionist. When you're working in a way where you can't go back, it forces you to be innovative. If there's a "mistake" you have to find a way to make it not be a mistake anymore...and when you know that you have that ability, you're not afraid of it happening. So you learn to just plunge in to the picture without worrying.
im really into the ctrl+z stuff. lol
i never really thought of it that way though.
ill have to keep that in mind when im working with something off of the computer,
pretty cool