If you are going to use 2ft (18w) floro tubes, a minimun of 4. Up to six. Colors, better that cold lights dominate. Three 860 or 865 (daylight) and one 830 (warm white) will do a good mixing (if you use 6 tubes, 4 daylight and 2 warm white). But any color works if you cant find the most appropiate. Only make sure you use triphosphor tubes, they last much longer and have better efficiency.

Using electronic ballast is a good invest, your tubes last much longer and saves watts, for a little more initial cost.