Back when you didn’t get weird looks for watching Darkwing Duck, cassette spacers were rarely needed. If you had a 9 speed cassette and a 9 speed hub, bam, they worked together. Then 10 speed and 11 speed stuff came along and upset the canoe, leaving all your picnic ingredients soaking wet and your cassettes suddenly loose and wobbly, like a jelly fish at the disco. Once 10 speed and 11 speed cassettes and hubs were released, people threw spacing standards out the door. Some hubs needed spacers, some didn’t. This problem persists to this day. We routinely fit 11 speed cassettes onto 11 speed hubs and have to shim the cassette with a spacer to make it snug. Lame, but #truefacts.
If’ns I were a home mechanic, I’d keep a small assortment of cassette spacers around. Ya never know when you might need one. Put the biggest one you can on while still being able to fully thread down the cassette lockring. Careful not to strip said lockring's threads!
These spacers are only for 7 to 11 speed Shimano style freehub bodies. They won’t work and are not needed on SRAM XD freehubs, and they won’t work on Campagnolo hubs. It’s hard sayin’ which one you will need for any given cassette/hub interface, so buy ‘em all and be glad you did. Sometimes you need a few: If you want to install a 7 speed cassette on an 11 speed hub, for instance.
The color doesn't matter, you can't see this little nugget. It's gunna be either silver or black anodized.