White-tailed Deer in temperate regions do, indeed, change color when they shed, for two reasons: camouflage and thermoregulation. Normally, the summer coat is reddish brown and the winter coat is buff or gray. Gray is less visible against bare trunks and branches and helps the deer absorb heat. Deer in agricultural areas exposed to lots of sunlight tend to be paler than those in forests. The fall molt usually begins in August in your area.
I'm afraid I have no explanation for the dark deer you've been seeing, but it doesn't make any sense to me that it's somehow a predictor of coming conditions - unless the deer are sensing a future with no environmental regulations and lots of industrial air pollution! I know I am!