If you want a one shot and get it over with then you need to do the following:
If you cannot successfully complete an alternative color plate test, you will be required to pass an operational color vision test (OCVT). This test has two components:
(a) A signal light test administered at an airport air traffic control tower; and
(b) A practical test in which you must read and correctly identify colors on aeronautical charts.
Upon successful completion of both elements of the OCVT, the aviation safety inspector will issue a letter of evidence and a medical certificate with the limitation “3rd Class Letter of Evidence.”
If you fail the signal light test portion of the OCVT during daylight hours, you will be able to retake the test at night. If you pass the nighttime test, your medical restriction will read, “Not valid for flights requiring color signal control during daylight hours.” If you cannot pass the OCVT during day or night hours, the restriction will read, “Not valid for night flying or by color signal control.”
Important note: If you fail the daytime signal light test, you will not be eligible for either first or second class medical certification, may not be issued a letter of evidence, and may not have the limitation modified or removed.
For first or second class medicals:
(a) Successful completion of an operational color vision test (OCVT) described above; and
(b) A color vision medical flight test (MFT). This is an actual flight test and requires the following:
(1) You must read and correctly interpret in a timely manner aviation instruments or displays, particularly those with colored limitation marks, and colored instrument panel lights, especially marker beacon lights, warning or caution lights, weather displays, etc.
(2) You must recognize terrain and obstructions in a timely manner; select several emergency landing fields, preferably under marginal conditions, and
And if you pass all of the above then you will get a LETTER OF EVIDENCE and not have to take the tests again.