Several online versions of the Ishihara test for color blindness exist, but it's best to take the test in a controlled environment such as a doctor's office. The test consists of 24 slides with a collection of differently colored dots arranged to form a number visible with proper color vision.
Results of a self-administered online test may not be accurate because different monitors have different color settings; differences in ambient lighting can also throw off the results. Taking the test in an ophthalmologist's office removes these variables. A trained ophthalmologist can diagnose a subject's level of color-blindness based on the test results.