Nu's pretty right, by my experience. White is by far the most useless color of the five, yellow works rather well with any non-sex class, blue works with any 'educational' (i.e. non-joy) class, red works well except with math, econ, and moral, and green is bad for math/econ, good with 'joy' classes, and neutral for the rest. My normal strategy is to combine blue/red for both of the 'doubled' sections, then combine yellow and green, and leave the whites on their own - go heavy on the blues and yellows to start, then slowly use red and green more often as you gain more... 'influence'. Ignore white entirely. Of course, this is only one strategy; there are many others, and a couple that work equally well in all situations plus some that work in specific cases.
EDIT: and I've normally found that while obedience class is a nice thing to get, my normal gameplay just makes it too damn hard. I never get more than a handful of the required 15 dots.