If the Ender Dragon enters a space where there is any block directly above it except for air, its texture will be rendered much darker than it is supposed to. This problem seems to be in effect regardless of lighting conditions, as I found that it happens both during the day and at night, even in a well-lit area. If the player has the Night Vision effect applied to them, this issue does not occur.
I have enclosed screenshots so that the situation can be seen properly. When the Ender Dragon flies out from below the blocks, it is rendered properly, but when it goes back under, it is not. The issue appears to only be with the "dragon.png" texture, and not the "dragon_eyes.png" texture.