Try the following:
Check Settings > General > Accessibility >Grayscale. If it is turned on, just turn it off.
Some readers found these tips also helpful:
Fix all the icons at the same time by turning off the Passcode Lock, reset iPad, turn Passcode back on.
Reset Network Settings and then Restart the iPad (Settings > General > Reset > Reset Network Settings
Reset All Settings (Settings > General > Reset > Reset All)
What I did to resolve the issue is put the affected apps into a folder and the icons come back. Not sure why, I guess it refreshes them or something.
Hope one of these fixes your issue!