It depends on water height and clarity most of time (like in 95% of the time). When the water is low and clear, it is time to use the smaller stuff (as in #6 and smaller) and also time to put the skaters to use. When the water is high and cold, it is time to put on the big stuff. And most of the time it really doesn't matter the color or brightness of the fly used. Size matched to water height, temp, and clarity with good presentation is what matters most. However, if I get a grab but no hookup on a small fly in summer/fall low-water time, I go back through with a fly at least 2 sizes large that has a different silhouette or change from a skater to a wet simply to give the fish something quite different to look at. Sometimes this provokes a strike, sometimes it doesn't. And in winter when the temps drop and there hasn't been rain for a while when the rivers drop and get clear, but the water is rather cold, I use smaller flies and fish the in shallower water because it is warmer there and the fish a bit more active.