What is the definition or description of: impaired color vision?

Below normal tests: Color vision is tested by a variety of standardized testing devices and plates. Impaired color vision is when there is failure to achieve a score of normal on the testing device used. Most color deficiency is inherited but may also be caused by advanced retinal disease such as in diabetic retinopathy, and it may even be caused by certain brain strokes termed cortical color deficiency.