What follows, is a direct quote from Foundations of Graphic Design, an old Design text book I found recently, definitely an interesting exercise

The Ishirian test, developed in Japan, is administered internationally to determine colorblindness. If you read the number 74 in the above right circle, you have normal color vision, but if you see the number 21 you have a red green deficiency, the most common form of colorblindness. If you are colorblind, you will see the number 2 in the lefthand circle, and if you have normal vision, you will not be able to clearly distinguish any number.