I have painted several guns for predator hunting. As to if it makes a big difference or not I'm not sure. I do like the fact that a painted gun is much less likely to rust. I have alway used the krylon ultra flat paint. Its easy, cheap, and really quick to touch up.
I was predator calling one time in the dead of winter, was using a black synthetic stocked Ruger 10/22mag when a fox came in to the call. It was about 60 yards out and moving thru some brush so I sat up a little and held up the rifle waiting for it to come out the other side. As soon as it cleared the brush it lit out like its tail was on fire. I couldn't figure it, I hadn't moved and was very well camoed up with snow camo, there wasn't any real reason it should have spooked, the only thing I could figure was that the all black rifle stuck out like s ore thumb against all that white. I painted the whole rifle in a snow camo that night and then waited a week for the paint smell to wear off. A week later I called in the same field and the fox came in and I shot it. Did it happen because I had the rifle painted up? Not sure, but I know it doens't hurt anything.
If you don't want to pain your rifle you can do an excellent cammo job using white "vet wrap" a bandaging wrap generally used for horses, it is rubberized and only sticks to itself. Here are a couple of pictures of some of my predator rifles.
