Birds' Vision
The first thing to know is that not all birds' eyes are the same. You probably know that owls see phenomenally well in the darkness, that hawks see well from great distance, and now you have read that songbirds see bright colors extremely well. All these birds' eyes have different features.
We are focused on songbirds, because those are the types of birds (primarily) that your outdoor cat will likely hunt. Some cats will hunt the occasional woodpecker, ruffed grouse, or other non-songbird, and those non-songbirds' eyes are different in terms of color perception.
Back to songbirds. These are birds like warblers, phoebes, robins, and chicadees, for a few examples.
1- Songbirds' eyes perceive bright colors extremely well due to having more rods and cones that human eyes. They also have "double cones" which are specialized to see colors, as well. Bright colors seem even brighter to their eyes, and that makes the colors stand out in the environment more so than they do to your eyes.
2- Songbirds see bright colors well during low light conditions (because of having more rods). This applies to dawn, a time of danger--and singing--for many birds.
3- When a songbird perceives the bright colors of Birdsbesafe® cat collars, it seems to also perceive the predator. I suppose that if the birds' attention is drawn to the bright color around the cat's neck, it can at the same time perceive the shape of the predator.
It will take some scientist to confirm this, but Birdsbesafe® customers report that birds fly away from the cat immediately and from a distance.
For More Reading about Birds' Vision
Birds' eyes are very interesting and we include here a list of some interesting articles for your perusal.
An article with more about birds' rods, cones and sight. Not specific to songbirds.
An excellent short article about rods, cones, double cones.
A lengthy article with citations, but not very focused on songbirds. Has a good section on anatomy.
A compilation of most of the scientific journal articles on birds' visions. read and summarize
An article about night vision of songbirds being adequate to migrate in the dark, and corresponding area of the birds' brain.
Fun and lively article that mentions that birds of prey don't see color in the way songbirds do.