How the color vision screening works
Each plate is made of colored dots, and a number is hidden in dots of a slightly different color. People with typical color vision see the number easily. People with red–green color blindness see the two colors as nearly the same, so the number fades into the background.
The first plate is a practice plate that everyone should be able to read. If you miss it, the problem is likely your screen settings, not your eyes.
Screening vs. the full color blindness test
This quick check answers one question: is your red–green color vision typical? If you miss plates, take the full color blindness test, which uses plates designed for each type of color blindness to tell you which type you have (red, green or blue–yellow) and how strong it is.