By three years old, on average, a child starts naming colors with some accuracy. At this age, however, if you show Jimmy green and he tells you it’s red, he may be seeing the right color but mixing up the names.
With real color blindness, most people can’t distinguish between red and green. This condition can be diagnosed with a color chart by an eye doctor, but it’s hard to identify it before age four. In any case, there is no treatment, so early detection is not crucial and people adjust very well to this minor impairment.