Question

why is color blindness a sex-linked disordder

Answer 1

because it is on the X chromosome of a female

Answer 2

Males only have one copy of the X chromosome (they are XY). The mutation for color-blindness is on the X chromosome, so if the mother is a carrier and passes down the mutated X chromosome to her male child, he will be colorblind. On the other hand, if she passes down her unaffected X chromosome, the male will be unaffected. Thus, for males, becoming colorblind from a carrier mother is a 50-50 chance.
Girls have two copies of the X chromosome, so even if a carrier mother passes down her mutated X chromosome the girl will still have an unaffected X chromosome, and since colorblindness is recessive, the girl will be unaffected. It is possible for a girl to be colorblind if her mother passes down a mutated X chromosome and her father also does (which would mean that he's colorblind), but that is very rare.
As a result, almost all colorblind individuals are males, and colorblindness is sex-linked.