Is there a way to prevent iOS 7 from changing the colors of wallpaper images?
There is a toggle you can flip to increase this effect even further.

Settings -> General -> Accessibility -> Increase Contrast

I'm a bit surprised the screen capture restores the original color, but I'm not surprised that on iOS devices with a powerful GPU that processing is happening on something as layered as the home screen / springboard view. That processing is one of the main design features of iOS 7 as I see it. Increasing text legibility on the home screen and the lock screen are clearly design considerations I would expect to see - even if they are subtle or slight as you describe for your current choice of custom image.