Windows 7 computer, Java version 1.7.0_21
I was playing in Creative and in the menu I noticed grass had a gray top. I placed grass, and then the whole grass part was gray (not the dirt though). Also affects leaves and tall grass, and doesn't only happen when you place the grass.
I noticed that this is caused by switching from a texture pack to no texture pack.
MC-15448 Gray grass when changing from a texture pack to the default pack.