Color is really all a personal choice. Lots of folks like Black. I have seen some a mottled black, blue and purple look and it too looked good. As for paint type use what ever is chepaest or most readily available. Regular water based latex paint is sufficient as is oil or lac. Brand or type is not important, nor is how it is applied, brush, roller spray etc, nor is flat, semi or gloss, although gloss is better if you spill saltwater on the outside at all it is easier to keep clean, but it all looks glossy from inside due to the glass.....Just beaware if the tank is acrylic, oi or lac based paints may start a premature crazing effect, and Fission / Fusion paint by Krylon is NOT recomended fo ruse on acrylic plastics.......nor is any paint really suitable for glasss overall or at least none thats sold in big box stores, as real truye glass paint is either baked on or has an etchant in the paint which is pretty well a haz mat item.......(hydroflouric acid IIRC) but fortunately most paints work just fine on glass for this purpose anyhow. My choice is usually flat or semi flat latex paint, applied by small roller.