Gray grass when changing from a texture pack to the default pack.

Grass is not colored/coloured...

Edit:@qmagnet "I can confirm this bug exists. I was about to add it but found it here first.
With various testing, here is what is causing it.
When you START minecraft in a non-default texture pack (I used Soartex Fanver), then switch to default, the grass texture is wrong (greyish in that case).
Also, the block is visibly wrong in the creative menu.
However, the grass loads properly if you start minecraft in DEFAULT texture pack, then switch back and forth.
But not if you start on a non-default texture pack.
Also, it's not always grey. It depends on the texture pack. With Sonic Reloaded started, the default grass was a much deeper green.
I have confirmed this bug in snapshots 13w18b, 13w18c, 13w19a."

I have added this as an edit,this is your quote,and you deserve full credit for explaining what has happened.