Is this the one?
You wear it like a ring. Slide one color into the upper loop and one into the lower ring. The coil keeps the colors separate so you can easily grab the one you need.
I like the metal one much better than the plastic one on the same page, which keeps the colors parallel to each other and makes it harder to pick up the right color with your needle.
Finally, here is Grumperina's homemade version, with a Bandaid wrapped around an old ring.
If you're still having a problem figuring out how to use one of these gadgets, ask for a lesson at the store that sells them.