Answer:
Because females have two X chromosomes and the red-green color blindness allele has to be in both chromosomes to cause the woman to be color blinded. In the case of males, they only have one X chromosome, so if the allele for red-green color blindness is in the chromosome, the male will be color blinded since there is no other X chromosome with the normal allele as in females.
Explanation:
What determines if a person's sex is the genes X and Y. A sperm carries one of these two genes. When it meets the egg, this one will give the other chromosome, which is always X. So, if the sperm carries the X chromosome, the result, when the egg's fertilized, will be XX, which will give the information to develop a female human. If the sperm carries the Y chromosome, the result will be XY, which is a male.
As females are XX, if the red-green color blindness gene is in one of them, the other chromosome, which does not have this allele, will prevent the woman from being color blinded. We can say that women have a backup copy that prevents color blindness.
In the case of males, as they are XY, there is no extra copy that can prevent them from being red-green color blinded, so when a male has an X chromosome with this allele, they will be color blinded. In the case of women, they will only be carriers of the gene and will be color blinded if the two X chromosomes have the red-green color blindness allele.