some entities (certain signs and banners) are lighted up more than others when not in F1 mode.

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.