Doctors most commonly test children for color blindness using the Ishihara Color Test, which involves a series of circles formed out of different-colored dots, explains the National Eye Institute. The dots form shapes inside of the circles that individuals with red-green color blindness are unable to see clearly.
Some people with red-green color blindness can see the shapes inside of the circles on the Ishihara test with difficulty, while others are unable to see them at all, according to the National Eye Institute. The Cambridge Colour Test is another option for testing children for color blindness. During the Cambridge test, the letter "C" randomly appears in four different locations on a computer screen, and patients must press a button that corresponds to the screen location each time they see the letter.
Doctors also sometimes use a device called an anomaloscope to diagnose color blindness, notes the National Eye Institute. Patients see a two-toned circle when looking through an eyepiece on the anomaloscope. The top half of the circle is a yellow hue, and patients can adjust the brightness of the color by turning a knob. The bottom half features a mixture of red and green light, and patients are able to adjust the proportions of the two colors using a separate knob. Patients receive instruction to turn the two knobs until both halves of the circle appear the same color and brightness.