Once we got a large batch of stone, and I asked the contractor "why'd you get two colors?" He looked at the angry clouds and told me he would take care of it right away. Sure enough, five minutes into the rainstorm, he had already swiched the light ones out to the darker gray. I told him I liked the lighter gray and he said "check back in a day". The guy is a miracle worker. I'll ask if he does roofs.
Seriously, you're looking at an area of the roof that is dryer than the rest. Or it may have a crust of frost moreso than the rest of the roof.
Parts of the roof dry/melt at different rates because of differences in what's under the roof. While this can be a "canary in a coal mine" warning of a problem, it may also just be normal. Canaries in coal mines also die for other reasons.