There are also a few individuals who have no cone cells and are completely colour-blind. We all have rod cells, which provide vision in low lighting and are not sensitive to colour. This is why we see the twilight world in shades of grey. People with no cone cells see the world this way permanently. Women have two X-chromosomes, so if a colour is missing on one they have a spare.
Men have only one X-chromosome and so are much more likely to be colour blind.
Her son, who has only the two green pigments, is colour blind. But the woman herself may have cone cells for blue, red, green and the extra green. Instead of having the usual three dimensions of colour she might have four. She would be a tetrachromat. Drs Gabriele Jordan and John Mollon of the University of Cambridge have been trying for some years to confirm the existence of tetrachromats, with good theoretical reasons.