How Does an Ishihara-type Color Blindness Test Work?

When taking the Ishihara test, the object is to correctly identify the symbol (usually a number) shown within the Ishihara test plate. Test plates are designed using a special random dot pattern, whose colors and sizes create what’s called Luminance-Contrast Noise.
The resulting pattern of colors camouflage any color brightness differences (Luminance Contrast) that might help the color blind detect and identify the number.
That means that to read the symbol hidden in the plate, you are forced to use only your perception of color. Additionally, the colors used to camouflage brightness are highly desaturated colors.
That makes it difficult to distinguish between the colors, enabling only those with normal or typical color vision to see the hidden symbol and pass the test.
And, because of how the color sensitivities are shifted in the color blind (see our blog: What Do Color Blind People See?), these desaturated colors confuse the color blind, and make numbers in the Ishihara test invisible.
Ishihara is a good test because desaturated colors confuse people in real life. For instance, pink, light green and grey are more or less identical to the color blind.