I've wondered how important the color is in many situations. Last fall we went out for steelhead (Lake Erie streams) for the first time. Fish kept hitting nymphs regardless of color. They faithfully ignored all the blood dots, sucker spawn, etc. I was just amazed that they'd pass up those tasty looking triumphs of fly tying I had produced in favor of those tiny drab looking nymphs. I don't know that anybody tried yellow, but everything from a sort of tan color to black appeared to work. I concluded that what looked good to me wasn't what mattered.