That is a very good questions. The answer lies in the genetics of sex (I mean male and female). You probably know that there are two sex chromosomes; XX for females and XY for males. Certain conditions including colour blindness are carried by the X chromosomes. The Y chromosome does not carry these inherited traits. For colour blindness, there is usually a mutation in the X chromosome. The girls inherit one X chromosome from each parent. If there is a mutation in one chromosome, it is covered by the fact that the other X chromosome is normal and they will not have the disorder. For boys they only have the 1 X chromosome from mom. If that is faulty then they will have colour blindness.