With the new iOS 7, the contrast is so much better with white letters on dark backgrounds and black letters on light and white backgrounds (the best). The worst are white letters on light back grounds. See below for examples on some of the new wallpaper backgrounds.
Pressing on your home button 3 consecutive times will change background to black and letters to white for improved contrast and better for viewing at night. See last image as an example. Repeat the 3 clicks to return to normal.
Article and images created from my iPhone 5.