If your fishing during daylight and using a floating line, color really doesn't matter. A fish will likely see the silhouette of the line against the sky and not the color. Thus the idea of tippets. If you're using a sinking line, that's a different story. Nice dark colors, or colors likely to match the water conditions and bottom color. Colored floating lines do help you see them, but mostly when I'm casting, I tend to look at my target, not the line. As well I never take my eyes off that fool fly once it hits the water! LOL