Colour blindness is an X-linked disorder. For colour blindness to be effective, all X chromosomes within an individual must be affected, and since men only have one X chromosome and women have two, the chances of men being impacted are exponentially higher.

However, in this case, the mother is colour-blind and hence she carries X^CX^C genotype. Father’s mother is also colour-blind and hence even she will be having the genotype as X^CX^C, and she will be transferring her X^C chromosome to her son and hence, even the son will be colour-blind (X^CY). So, by this, we can tell all the X chromosomes are coding for colour-blind gene, and they inherit this gene to all their children, and hence, the chances of their child being colour-blind would be 100%.