Best advice, if you can distinguish between red/green/white, check out the FAA website, and find the closest regional office, and find out if they have a farnsworth. If you passed the farnsworth in the past, shouldn't be a problem again. Other option, is to find one in your area. Once you pass, like the doc said, you're issued a letter, and that's it. Just make copies for your AME, and you won't have a "Not valid for night flight or by color signal control" restriction on your medical. I would say go for it, especially since you've already tested with and passed a farnsworth already.
I took the farnsworth a few months ago. I passed, got my letter, and renewed my medical and the restriction was gone. They told me that if I failed the farnsworth (where I took it), that the restriction IS NOT permanent, rather you just have to find another farnsworth because they wouldn't let me retest. All in all, go for it, you have nothing to lose.
Best of luck!!!