Why is colorblindness more common in men?

It is true that the most common type of color blindness ("congenital red-green color blindness") is much more common in males than females.

This has to do with the genes we inherit from our parents, which are contained in molecules called chromosomes. The genes that cause the most common type of color blindness are found on the X chromosome. Males get an X chromosome from their mother and a Y chromosome from their father, while females get an X chromosome from each parent. Males only need to have one abnormal gene on their one X chromosome to be color blind, while women need to have the abnormal gene on both of their X chromosomes.  If a woman has just one abnormal gene then she will not be color blind, but there is a 50% chance that she could pass the abnormal gene on to a son who will then have the condition.