When placing End Stone beneath End Portal Frames, you can see that the End Portal Frame texture is much brighter than the End Stone, although it is the same texture.
Yes, I mean in the default texture pack.

This probably is because of the fact that the End Portal Frame is actually transparant, while Endstone is not, so this would be in every texture pack, unless the artist of the texturepack tries to match it with the brighter color.

How to reproduce: Look for a completed end portal and place end stone beneath the Portal frames.