Birdman, IJ has some great advice in his post, especially about feeding stations. I fished for several years in the park (about 15-20 times, persistence or stupidity?...) with no fish, myself. What finally helped was hiring a guide and fishing with him for a day. He helped with a number of subtle things like feeding stations and being sneaky. Catching fish with him really helped my confidence, and from that point forward I was able to regularly catch fish in the park.
If you can't afford a guide or want to do things on your own, there are a number of things to consider. As IJ mentioned, water temp is crucial. I may even take it a bit further. While the fish probably won't be looking up until the water is 50 degrees, they may not be really active feeding on top (taking dries) until the temps start approaching 55 degrees.
Also, the fish at Metcalf Bottoms and Tremont are used to seeing humans and flies, so they are much more sophisticated than other streams. I think you might be better off going somewhere you might have to hike into, while you are getting started (above campsite 18 on the WPLR might be a good place). I think fly size is more important than color. If you want to fish with dries, try to find one pattern that you are happy with and stick to it (for the most part). A gray Parachute Adams or an olive Elk Hair Caddis are good choices right now. Try to observe the size of bugs coming off the water and approximate the same size (or slightly smaller version of your fly). And then don't change-keep fishing. Work on your stealth skills, presentation and drift. Too often folks are standing around in a stream changing their flies and not fishing. The more time you have a fly on the water, the better your chance to catch fish...
It is very important to be sneaky. I am constantly amazed how often you can catch fish when you are hiding behind rocks or overhanging branches. If you can see the fish, they can usually see you. If they can see you, they are very unlikely to be caught. Fish upstream as much as possible, try not
to backcast, stay low, wear muted colors, move slowly...Have fun! If it gets to be a chore, you won't catch fish...they can sense it