Listen to the Fish for Crankbait, Topwater Color Selection
I can't tell you why one color of lure works better than another on any given day. What I can tell you is having the right color tied on will help you land more fish. This seems to be especially true when fishing topwater and crankbaits.
Let the Fish Tell You
Let's say you've caught a few fish already on a squarebill. However, the fish you've caught have been hooked on the side of the mouth and you've lost a few too. This likely means that your crankbait is the right choice but the color you have tied on isn't quite what they want. When you're hooking fish with the front treble hook or the lure is well inside the fish's mouth, you have the right color tied on.
I can remember coming up on a school of spotted bass at Table Rock Lake. I had a sexy-shad-color spook tied on and had a couple of blow ups and lot of fishing following it back to the boat. It was a sunny day and I knew something wasn't quite right. The fish were getting too good of a look at my spook. When I switched to a chrome spook, I started catching a ton of fish in a hurry.
Fine tuning your color selection and letting the fish tell you what they want will mean more fish in the boat when fishing these presentations.