Resin glands and hairs are usually a good sign because they indicate that the plant had ample nutrients and had excess energy to expend, both on producing the glands/hairs and on producing more THC. I've heard mixed reports as to whether the resin glands themselves have high concentrations of THC; I suspect that it varies by strain, with some strains (such as those grown to be used to make hash) having THC-rich resin and others (probably including most strains) having resin that contains mostly other cannabinoids. I'm pretty sure I read somewhere that scientific studies have shown no correlatin between resin glands and potency when compared to less resin-heavy plants that had equal access to water, light and nutrients; however, as I said, the presence of lots of resin indicates favourable growing conditions, so in practice, resin glands are a good sign whether there's a lot of THC in them or not.

As for colours, I doubt there's any genetic correlation between darkness of the bud and amount of THC (although I could be wrong). A more likely explanation for any observed correlation is that a majority of the good strains in your area happen to have darker buds. The best I've ever smoked was pretty average in colour, but covered in resin glands.