Watts Per Gallon was only really applicable with standard tanks and old T8 bulbs. Since you're using a modern fixture, it's something you should completely ignore.
and read through the lighting sticky. Pay close attention to the thread about measuring light with PAR.
Bulb color is all up to you. 6500/6700k seems to be the basic daylight bulb most in the planted tank hobby use. Some use 10,000K and 14,000K bulbs because they like less yellow/more white lighting. But there are all kinds and colors of bulbs available on the T5HO market.