What route are you taking? Will you only be in CR and Panama, or are you going to other countries in Central America as well? Two years ago I rode from Cancun to Panama City from Oct to Dec. It was the hottest and most humid riding I had ever done. Get the most breathable shorts you can find, because you're going to sweat. There was so much sweat pouring off of me that puddles formed under me each time I stopped to rest. While riding, the sweat saturated my shorts and dripped off of them onto my calves with each pedal stroke. It was crazy. Got dehydrated despite drinking tons of fluids. BRING HYDRATION SALTS. They saved me. The worst for heat was Mexico and Belize, but it could be hot anywhere down there.
Black may not seem to be the best color for shorts in hot weather, but when it's as humid as it gets down there I don't really think it matters much. I had tan shorts that faded with time, and had the negative effect of turning darker in the area that came in contact with my black seat, which wasn't very fashionable, to say the least. They are nylon and despite trying everything I could think of to clean them, they resist. I'd go with black if I had it to do over again.