To see in full colour as we know it, humans use three cones – red, blue and green. However cats and dogs only have blue and green cones. This means they have a much more muted perception of colour, which is akin to colour blindness in humans. As cats and dogs are not sensitive to red light, they have difficulty distinguishing some colours. For example, behavioural tests in dogs suggest that they can distinguish red from blue, but often confuse red and green. Similarly, they often perceive green as grey.