Color blindness is a recessive sex-linked human trait. If a color-blind father and a mother with normal color vision have a color-blind son, what must be the mother's genotype?​

Remember that most of the time, in biology class at least, the sex linked disorders are usually going to come on the “x” chromosome. Colorblindness does come on the “x” chromosome.

To solve, begin with a key, such as:
colorblind: c (since it’s recessive)
not colorblind: C

Remember that males have XY, and females have XX, also remember that sex linked traits are written as exponents, and sometimes as subscripts

The father is colorblind, so his genotype is X^cY

Now you must work backwards in a punnet square. Put the fathers genotype on one side of the square. You don’t know the mothers yet, so for now, just put her two x chromosomes. Remember that mom can only give an x to the baby, because that’s all she can possibly give. Since the child is a boy, Dad will give his “y” chromosome to the child.

If the child is colorblind, as the question states, then moms “x” chromosome that she gave the child must have had the recessive allele in order to give him colorblindness. So that means that mom carries the recessive allele. However, mom has normal vision, so she must have a dominant allele as well, otherwise she would be colorblind.

Moms genotype is X^C X^c

I know this is very long winded I’m sorry :(
But I do hope this helps!