That disk-like looking sanding "wheels" on the chart are actually round pieces of sanding paper. Thick disks (which are usually attached to the mandrel permanently) are for grinding, not sanding.

The color of grinding stones is different because of different abrasive material used. Meaning? I'm usually just trying different stones on my projects and see how they behave at different speeds. Some are too soft, some - too coarse, you will see that at once. The strongest and sharpest grinders are greenish colored. Pink seems to be the softest. Not sure about white stones.