The 'gene' which causes (inherited, red and green types of) colour blindness is found only on the X chromosome. So, for a male to be colour blind the colour blindness 'gene' only has to appear on his X chromosome. For a female to be colour blind it must be present on both of her X chromosomes.
Because since their ancestors came before them then they inherit certain beneficial traits that help them survive. Those traits are in the DNA and the animals are evolving to be more fit for the environment that they are in. (hope this helps)