I'll try to explain it as simply as I can... Since there is no exact/specific effect that comes from an indica or sativa it is a difficult thing to answer completely accurately, like richardskinner said. Hmmm, I'm trying to think of the way to word this best without writing a book... lol.

Basically a sativa that gives you a very cerebral/soaring effect will always give you that type of effect, and same with an indica that gives you a very stoney type effect. The coloring of the trichomes does not affect the way you feel when you use it as much as the genetics themselves. So the goal should not be to harvest in such a way that youre trying to change the effect of the plant, such as make a high flying sativa give you a stoney couchlock effect, because you'll likely harvest too late and ultimately just have reduced potency. Your desired effect should be decided based on strain and genetics, and then you play with harvesting at different times to slightly alter that effect to fit your needs perfectly, as well as achieve maximum potency. There are several good threads here in "Basic Growing" about how cannabis maturity actually works if you want to learn about it more specifically.

If you have any further questions or if I didn't explain that well enough feel free to ask some more questions to have me clarify, otherwise good luck with your harvest!