Purp... here is what I do to determine when to harvest.

First of all, it has nothing to do with the color of the pistils. I have seen many recommendations out there to pull when 50% of these pistils turn brown or red or whatever, but the problem is that this is strain dependent. Some strains will color the pistils deep reds and purples from early in flower... it has nothing to do with the ripeness of the buds. Also, in many strains, these pistils start turning brown way before the trichomes have fully developed.
The THC is in the trichomes, some of them on the pistils, but so much in the pistils. Watch the trichomes, to see when it is time to harvest. If you want a more cerebral high, pick when the trichomes have all turned cloudy and very few have gone amber. A more physical high will be had when the buds are a bit riper, when some of the trichomes have turned amber, and most people harvest anywhere from 5%-50% amber. The problem is that after amber is red... and the deeper colors mean the trichomes are beginning to break down because of the light and you actually start losing potency. Let your plants go too long and you will miss the peak.
You really need a small microscope, loupe or a camera that can do macro mode to see these trichomes well enough to make this determination, but it is well worth the effort. Try some buds at various stages of trichome development, and see which type of high is the one that is right for you with the plant you are growing... its is always a personal decision and can not be made by us out here.