Colorblindness is more common in males than in females because it is considered to be a sex-linked trait. In this case, the gene for this trait is located along the X chromosome, and males tend to inherit X-linked conditions more frequently than their female counterparts.
Males have only one X chromosome, which they inherit at fertilization from their mother's egg. They inherit a Y chromosome from their father's sperm. Because they only possess one copy of the X chromosome, they are said to be hemizygous for any genes located on the X chromosome. Therefore, if they inherit one copy of a normal vision gene on their X chromosome, they will have normal vision; if they inherit a colorblindness gene on their X chromosome, they will have a colorblind phenotype.
Females, on the other hand, possess two X chromosomes—one from each parent. If a female has two X chromosomes and each has the gene for normal vision, she will have normal vision. If one X chromosome has a normal vision gene and one X has the colorblindness gene, the normal vision gene will be expressed in the heterozygous individual because it is dominant. A female must have two colorblind genes—one from each parent—in order for her to have the colorblind phenotype. The chances of this occurring are low, and that is supported by the fact that very few females are colorblind.