The color test? If you are semi-color blind you have a good chance with that because it is a LOT easier to distinguish a bright colored light than those little dot thingies. If you are hard-core color-blind it may not matter but definitaly give it a shot. There's not much you can do to prep for this, other than memorize what the signals mean.

Smoking degrades your vision all around...if you smoke I would stop immediately and take lots of eye vitamins (E, A, beta-carotene, lutien) for 6-8 weeks before you try the light test. But don't OD on the A vitamins, they can be toxic.

I had to do actual light gun signals on a checkride once (MEI?) when the airplane xmitter failed in the runup area...got the "taxi back to the FBO" signal.