Why blue & yellow
Two cones, not three
Color vision starts with cone cells at the back of the eye. Most people have three kinds, tuned to short, medium and long wavelengths of light. The brain compares the three and gets everything from violet to red.
Dogs have two kinds. That makes them dichromats. In 1989, Jay Neitz, Timothy Geist and Gerald Jacobs tested dogs on colored lights and found two cone types: one most sensitive near 430 nanometers (violet-blue), the other near 555 nanometers (yellow-green).
What that looks like to him
- Blue looks blue.
- Yellow looks yellow.
- Red, orange, pink and green collapse into the same muddy yellow-brown-gray, told apart mostly by brightness.
- Blue-green looks close to gray.
Dogs don't see in black and white. That one's a myth. They see color from a smaller box of paints.
Why a red ball vanishes
Grass green and toy red both come through as yellow-brown. To him, the red ball isn't a different color from the lawn. It's a slightly different shade of the same one. A blue ball is a different color entirely. That's the whole reason this company exists.
How we make the dog-vision pictures
Our "He sees this" photos and the hover on every toy use one filter: the Machado, Oliveira and Fernandes (2009) model of deuteranopia, a form of human red-green color blindness, at full strength. People with deuteranopia have the same two-channel color system dogs have, so it's the standard stand-in.
It's approximate, and we label it that way everywhere. It shows color only. Dogs also see less fine detail than you do, and they see better than you in dim light. No filter shows that.
The red toy on the toy page
Every toy sheet has one red tug labelled "Control. Not sold here." Turn on dog vision and it goes lawn-brown while the rest stay put. Without it, the hover would prove nothing.