That is a very good question. I will be interested to hear people’s responses. I can't say that I have ever noticed a difference and tend to use eyes that compliment the overall coloration of the fly I am tying but have used the red eyes to contrast at times. I think it will be interesting to see if there is a correlation between the color eyes that work and their corresponding visibility under water. Never read anything concrete but I believe some hypothesize that chartreuse flies work as it is one of the colors that is least dissipated under water. I believe red is one of the first to go which might make those red eyes look more like one giant pupil if fished at any depth.