Colour blindness test. Card with a central colour and four peripheral colours being used to grade colour vision deficiencies. One colour is similar to the central colour, whereas the other three colours are typical isochromatic confusions indicative of protan (red), deutan (green) and tritan (blue) deficiency. The hand is pointing to the matching colour, indicating that the person has normal colour vision. Colour vision defects may be inherited or can be caused by optic injury or nerve diseases. This test is called the City University colour test because it was developed by researchers at City University in London, UK.