Do an experiment yourself. I did one back when I was a teenager. A buddy and I had a bet one season on who would kill more game. Him, wearing the new (at the time) RealTree camo; me, wearing my grandpa's brown cotton duck hunting gear. We hunted everything we could: deer, ducks, rabbits, quail, doves, squirrels. In the end, we basically tied. We both got 2 deer. He got more ducks, I got more of everything else.
I think it's more important to practice good woodmanship. Use the wind, the shadows, available cover, etc. Like some of the others, though, I've had more really close encounters while wearing a leafy suit than I ever did while wearing solid colors.
The most unnerving experience I ever had, though, was when I was wearing a dark green Filson jacket and my brown waders. I was in Alaska, on a moose hunt, and a big brown bear took a dump less than 15 feet from me. I was sitting with my back against a huge rotten stump overlooking a river bottom, and he came strolling down a trail to my right. I was in plain view of him and was so focused in the opposite direction I didn't hear him come up. His breathing caught my attention. He drop his load, which was massive by the way, and moseyed right on by me. I guess I blended in just fine. Thank goodness.