Yes, unfortunately ATC (the initial medical) offers very little in color vision options. The pilot medical has something like 9 approved tests. To the best of my knowledge the ATC initial medical allows the Dvorine, and the Dvorine only. They had created a 'modified' Farnsworth that consisted of 27 pairs of lights (as opposed to the old version of 9) that was specifically directed toward ATC. Even though it was only done a handful of years ago, I do not think they allow it anymore. Also, they use to allow a 'strip test' that was administered for enroute facilities -- basically, you sit in a dark room and have to decipher various fonts of red and black on flight strips. However, I believe that is also gone.
Good luck on the medical. Other than the color vision test, it should be straight forward. (note: the cv test had me so freaked out that my rapid heart rate on the EKG set off flags by the white coats in OKC - had to go to my family doc and have him check my thyroid....which was just fine btw).