SmokinMoe,
Small and medium shrimp are definitely easier to smoke-cook becasue they are, well, smaller... That being said, they are not so thick that they don't cook fully through by the end of the smoking time. Large shrimp are more problematic because they are much thicker at one end, but still taper down at the other. Smoke long enough to cook them through at the thick part, and they will probably be rubbery and chewy further down the tail.
My advice is to cold smoke large shrimp. 40-60 minutes of cold-smoking will give them a wonderful flavor and color, but they will still effectively be raw coming out of the smoker. Then, heat up some olive oil/butter in your favorite skillet and pan saute until cooked to your desired amount. Since the saute will only take a couple of minutes, you can control the process precisely, watch them carefully, and pull them from the pan when they are perfect without all the guesswork.
As to using the color as a guideline, I don't think that is your best bet. Different varieties of shrimp will change colors at different rates, so depending on what you get from the store each time, you may be trying to reach a degree of color (or lack of color) that is unobtainable without turning the shrimp into a pencil eraser.