the gray is the thin one regardless.
They used to come in white! :P

The pink is usually the thick one.

The gray has 2 sides you peal, the pink only has 1.

On the pink i noticed its best to use the white side on the mosfet, and the pink hard side on the block directly. (should you ever have to use the pink one).
The gray is a standard tim pad, and i use it in everything outside of waterblocks as well.

The pink is there when you have a gap.
Not all gpu's will be 100% perfect.
You might have something a tad bit lower then what its normally at.
In this case you will use the pink one.

B4 you hard mount the block, make sure you lay the pads out, and then apply slight pressure where the pads are.
See if you get a imprint.
If not then you need to upgrade your pad to the pink one.

But if u dont touch the pink one, dont freak out, koolance just gives you "excess" incase you need it.