However, if they compare such colors as moss green, snuff brown, dark purple, and dark grey, all these color may look more or less the same. Small deviations from normal affect only some specific working conditions. That is why colour vision is examined at school before students get advice in career planning. The screening examination uses pseudoisochromatic plates. Most commonly used test is called Ishihara's test.
Screening tests are very sensitive and detect even minor deviations from normal color perception.
They do not measure the degree of deviation. For the diagnosis of deviant color perception another test is necessary, a quantitative test in form of small caps with color surfaces in all colors of the spectrum. The diagnosis of color deficiency should never be based on a screening test. If a child seems to have any confusion with colours, colour vision should be examined carefully. We have two eyes but see only one picture, image .