I keep 4 or 5 rods ready with different (usually my favorite) colors. Rotate through those different colors while fishing, if they don't produce like I think they should, I'll put on 4 or 5 other colors. Usually, the fish will give you some hints like you'll catch a few on a dark color, then you know to experiment with dark colors. Most lakes will have a few colors that are almost always productive, it takes time on the water (or a really good fishing friend that has spent time on the water) to learn these things. You can also ask at the local bait shop, but be weary, the "HOT" color may just be whatever the bait shop needs to sell.