Certain entities like signs and banners will be more lit up than other ones randomly. I noticed this when I saw two signs next to each other of different colors. When I pressed F1 to take a screenshot, I discovered that having F1 on turns the lighting effect off. originally when I thought it might be a new undocumented addition to the game to add random texturing to signs, I checked with f3 to see if it was saved in the NBT data, which it was not. After reloading the world, I discovered that the entities that got the lighting effect were almost randomly chosen. Placing a sign down also seems to have a random chance of being lit or not.

UPDATE:
mousing over items in your inventory also causes the lighting to disappear.