There were two types of yellow followers. The earliest had a 45 degree bevel at the bullet end. The second yellow follower did not have the bevel and looks just like the later white follower. Both of these followers are found in magazines that are stamped "ES" on the top front of the mag body about 3/4" below the follower.

The white followers are found in magazines that are stamped with "ES1"
Most, but not all, of the ES1 magazines have "Accu Guide TM" and "Patent Pending" on the bottom left of the mag body. The ES1 mags also have indentations that run along the sides just below the top of the feed lips. These look like a stiffening feature for the feed lips to me.

All the followers work fine and are interchangeable.

New ones purchased from S&W ($3.25 each) are white.