Been there done that. Very simple to overcome.
First, fail the test at your AME in which he/she will issue a medical certificate with a restriction from flying at night and/or depending upon how detailed he is place a restriction prohibiting flying by lights from the tower (like you would have a choice).
Second, call the FAA and tell them you failed. The FAA will send you a letter instructing you to go to your local FSDO in which a person will stand with you on an X, call the tower and flash the lights. Green, white red. Then, assumning you can tell the difference, the person will hand you a letter stating that you have demonstrated the ability to tell the difference and in her/his hand will be a new medical with no restriction. Put the letter in the safe, bring it with you to your physical for the balance of your life, show it to the AME and skip that part of the test. The time between the call and receiving the letter is about two weeks if you call and bug them and 4 weeks if you do not.
If you are worried about the test, call the local tower on the cell phone, explain to them the situation, and they will flash the lights at you. You can even have someone else stand there and laugh at you and say, no that is green...........er blue.......er yellow......you can do this as many times as you want as you are not doing it with an individual from the FSDO.
My father, 66, has had cornea transplant and the AME simply sent his paperwork to the FAA. The FAA required a form to be completed by his eye doctor (two minutes) and they sent him his medical right away and a letter saying not to worry again. $60 for the doctor, $11 for FEDEX.
Despite what people say, my experience is that the people at the FAA medical section are extremely helpful and have always been prompt and happy to help. The only caveat is that I only require a second class medical.
Now when your shirt does not match your pants, nothing the doctor or FAA can do..............