A property professional with nearly 20 years experience of estate agency in Suffolk and North Essex. During that time I have carried out market appraisals on over 5000 residential properties and have helped over 1500 people successfully sell their home.
Therefore, a son from a color-blind mother (who receives the trait from his mother on the X-chromosome) will always be color-blind, regardless of the state of the father (who can only give his son a Y-chromosome).
Do not show them each colour individually, they must be able to see all of the colours at the same time. If your child shows signs that they are not sure whether a colour is red, green, brown, purple, blue or grey, there is a reasonable chance that they are red/green colour blind.