The X chromosome is the female sex chromosome and when you have a pair of X chromosomes (XX), then that would denote that the subject is female. Now when it comes to x-linked traits, for a female to be affected, she would have to have the allele on both chromosomes, or in this case, homozgous for color-blindness. (If it is found only on one, she would be a carrier, but she won't be affected.)
