A normal woman, whose father was colour-blind is married to a normal man. The sons would be :-

Colour blindness is an X-linked recessive trait. Let $X^c$ carry the colour-blind allele.
The woman is normal but her father was colour-blind. She received her father's $X^c$, so she is a carrier: $X^CX^c$.
The normal man is $X^CY$.
Cross: $X^CX^c \times X^CY$
Sons receive the Y from the father and one X from the mother: $X^CY$ (normal) or $X^cY$ (colour-blind) in a 1 : 1 ratio.
Daughters: $X^CX^C$ (normal) and $X^CX^c$ (carrier); none are colour-blind.
So 50% of the sons would be colour-blind.