Colour blindness, specifically red-green colour blindness, is a sex-linked recessive trait, meaning the genes responsible are located on the X chromosome, making it more common in males. 

In the case of X-linked dominant traits, a single copy of the dominant allele on the X chromosome is sufficient for a female to express the trait, as they have two X chromosomes. If a female inherits one X chromosome with a dominant allele, she will express the trait, regardless of whether the other X chromosome carries a recessive allele. 

Males with an X-linked dominant trait will always express it, as they only have one copy of the X chromosome. 

X-linked recessive traits are expressed in females only if they inherit two copies of the recessive allele, one on each X chromosome.