Red-green color blindness is X-linked meaning it cannot be found on the Y chromosome and women, in this case, can exhibit the state of heterozygous or homozygous while men can only be hemizygous.
Let Xc refer to red-green colorblindness. Note, c is recessive.
Let X+ refer to "normal". Note, + is dominant.
The woman has normal vision meaning she is heterozygous X+Xc
Her husband is colorblind. Since he is hemizygous, his genotype must be: XcY
The children this couple can produce we cross or fertilize the genotypes of the father and mother
XcY x X+Xc
The probability that the first child from this marriage is color-blind is a quarter