true that refrigerant is colorless, but the oil may not be, and especially not if the AC had work done to it (ie replace compressor, evaporator, any lines, etc). if there has been any kind of AC problem and the vehicle was taken to a dealer or a private shop or whatever, the system is going to be recharged with a small amount of flourescent oil that looks yellow with greenish tint. this is used to diagnose a slow leak and where it is coming from.
but the "green" of it can really only be seen with light that has a special filter over it (i think infared or something, whichever one makes it look deep purple), and some yellow tinted goggles or glasses. this will make the flourescent oil stand out. and without this equipment the leak would have to be really really bad for you to be able to see it with your own eyes.
Thanks for the imput I appreciate it
and im pretty sure the op has all the info he needs, and some he doesn't
last thing i'll say about this topic is that the cap neither prevents refrigerant from escaping, nor does it prevent dirt or whatever from getting into the AC system (that's what the check ball within the valve is for). it simply protects the service valve itself from getting dirt or grime or whatever ON it. it's no different than a tire valve stem cap. if it gets dirty it might be a little more difficult to service the AC, just like it could be harder to fill your tire with air if the valve stem got all dirty and stopped functioning properly. that's why i'd get it replaced sooner than never, or else you'll just end up forgeting about it.
just make sure you get the right cap for it if you order it from the dealer or buy it at a store. high side has a bigger cap, low side has a smaller cap. you can tell if you look at the other service valve and compare sizes