Bob, I did something just like that. A couple tips:
1. Go a lot whiter on the top than you'd think.
2. Go higher with the black on the bottom than you'd think. Corals cover it quickly.
Here's my tank build thread, and the painting is on Page 2. Look at the later pages, and notice how everything looks with lights on it... much different than a dry, painted tank.
http://www.reeffrontiers.com/forums/showthread.php?t=28126
I just used the standard glass prep techniques. I used Windex several times, with a new towel each time and when I was done, I think I wiped it down with rubbing alcohol to be sure, but I'm not even positive I did that. I started from the bottom up. I wanted to make sure none of the blue up against the black overflow (so the overflow wouldn't be so visually obvious), so I painted the black across the bottom and vertically up against the overflow, then created an arc from the top of the overflow to the edge of the glass, about 8" up.
Next, I painted the darkest blue with several coats, giving time for each to dry. Luckily, I did this in July also, so it dried very quickly in this 110 degree weather. The "tricky" part really wasn't so tricky in the end, and that's the fading from one color to the next. Basically, the overspray from each color creates the fading effect. Just paint a little lighter at the edges of each color.
When it was all done, and I definitely couldn't see any light through the paint (I was using the sun as a backlight to verify full, even coverage), I used the rest of the black paint to cover over the whole thing nice and thick. I then let the tank sit outside for a couple days to assure complete curing and hardening of the paint. I recommend lots of paint for durability. If this were one color and you scratched the back, it would be easy to paint again, but with this setup you'd always see the scratch from the front side of the tank as a different colored stripe, or something.
