I remember a video on youtube, where the fellow was tying up a beadhead egg type fly. He was using yellow and black, I think he called it a steeler bug or something like that. When asked why he liked to use balck and yellow he made reference a study that stated out of all the color combinations of food offered to trout that yellow and black were the most successful. I can't recall everthing the gent said about the study. I think the experiment was in a controlled enviornment so with regards to light availability at certain depths it might not apply as it does in the wild. I think the reason why yellow and black might be effective is that it represents both light and dark shades.