How can you get color blindness if neither parent has it?

Trauma. Color blindness can be acquired (rarely) by trauma to the brain or trauma to the retina of the eye. It can also come from degenerative diseases that affect the retina, like macular degeneration as well as metabolic diseases, like diabetes. Another possible cause is vitamin a deficiency.
Basic genetics. Either the parent has the dominant gene with limited expression, or it's a recessive gene.