If you fail the Color Vision Testing (ie if you're color blind):

I use the Ishihara pseudoisochromatic plates: Concise 14-plate edition: six or more errors on plates 1-11 will disqualify you. Your certificate will then have a limitation stating the following: "NOT VALID FOR NIGHT FLYING OR BY COLOR SIGNAL CONTROL".
For applicants for a third-class airman medical certificate who has defective color vision and desires an airman medical certificate without the color vision limitation, you can go to the local FSDO for a Signal Light Test and Aeronautical chart reading test (called Operational Color Vision Test (OCVT)).
For applicants for a second or first class medical certificate (which I do not give), you must pass the OCVT above as well as a Medical Flight Test (MFT) which is also given by the FSDO.
Again, if you do not plan to fly at night or by color signal control, you do not need to do the above tests with the FSDO. I will be able to issue your medical certificate with the above limitations.