Perception of colours in Vision is determined by special cells in the retina known as rods and cones. Depending on how sensitive the cells are to perceiving colours, some people maybe unable to differentiate between certain colours. However it is not true that they are unable to see all colours. The most common is Red-Green blindness but there can be other forms too.
There is no clear treatment of colour blindness but use of special contact lenses has been seen to help.