Go to the hardware store and get a cheap spray paint. Just mask off the top of the tank and the parts you dont want to paint and go for it. Really all there is to that.
Oh yeah, Black is best cause I think so. Meowzer taught me that.
Thanks, guys! I like black also, so that's what I'm going with! The nice thing about glass is if I really screw it up, I can just scrape it off and try again.
Here's a picture of my 125 gal tank. I did a dusting of white back and forth a few times and then coated it with a nice "underwater" blue. It was the medium priced spray paint from our local BIG hardware store. I cleaned the glass with acetone first and its stuck nicely so far.
Black is nice but I wanted to try and have my tank look like I'm viewing it underwater while snorkeling.