As the character is recesive and humans are diploid, the idividual must have both alleles with the information for red-green colour deficiency if she is a woman, or the only allele with that information if he is a man; this is because the gene for colour deficiency is located on the sexual chromosome X.
Therefore, if both grandmothers have the allele for colour deficiency but they have a heterozygous genotype, neither of the grandparets would have colour blindness, even though, they could give their offspring the red-green colour deficiency allele.
To sum up, 0 grandparents can be red-green colour deficient (but at least one must have the corresponding allele) and have a grandson/daughter with red-green colour deficiency.