Colorblindness is a recessive trait and is only carried on the X chromosome. Colorblindness occurs with a much greater frequency in men than in women because men only need a single copy of the gene to be colorblind, whereas women need two copies.
The gene for colorblindness exists on the X chromosome, one of the sex chromosomes. Females have two X chromosomes, but males have one X chromosome and one Y chromosome. Because the gene for colorblindness is recessive, as long as a normal copy exists on one of the X chromosomes, the effect of the colorblindness gene is masked, and the individual has normal vision.
In men, however, only one copy of the gene is present because men only have one X chromosome. The single colorblindness gene is enough to cause colorblindness in men. Colorblindness, therefore, is called a sex-linked recessive trait.