The most commons forms of colorblindness are genetic conditions, passed along the x-chromosome. People with an x-chromosome and a y-chromosome only need their one x to be defective to catch it. People with two x-chromosomes need both to be defective.
Most women have two x-chromosomes (XX), and most men have an x-chromosome and a y-chromosome (XY). That's why colorblindness is much more common in men than in women.
