How to determine if someone is color blind?

Testing: The most common type of "color blindness" is red-green color deficiency. Blue-yellow deficiency or true color blindness (achromatopsia) are rare. Color testing can identify which. The problem is a lack of certain types of cones or their opsins (light-sensitive compounds). Red-green deficiency can be adapted to fairly easily and is not very limiting, but the others can significantly impact vision.

Testing: There are tests on line that can give you an approximate answer. Your ophthalmologist has test plates for the definitive answer.