Why does my iPod Touch show Spotlight differently?
I had this happen when I ran a beta version of iOS (8.0 beta 3 iirc). A reboot sometimes fixed it, but it kept coming back. The next beta fixed it for me.

Try rebooting, and check your accessibility options for greyscale (I have it in the triple-click home button menu, which I guessed was the culprit). Try disabling all accessibility options.

If you really want this gone and the above answers don't work, try a factory restore.