A man marries a woman who had a colourblind mother and normal father. What percentage of male children of this couple are likely to be colourblind?

The woman is a carrier (XX^c) since she inherits a normal allele from her father (XY) and a mutant allele from her mother (X^cX^c). Since the woman is carrier, half of her sons would be colourblind. Note that each son would inherit X chromosome from the woman and Y chromosome from her husband. Since one of her two X chromosomes carries normal allele and the other carries mutant allele, half of the would be colourblind (X^cY) and the other half would be normal (XY).

When a woman is carrier for colour blindness, half of her sons would be normal and half would be affected regardless of her husband's phenotype.