Be prepared for all kinds of lighting conditions, get a grey card - Did you know your camera measures light in shades of grey? For your photo to expose correctly, your camera must first determine which grey shade is medium grey. And although your camera is usually pretty good at determining medium grey on its own, some situations will cause it to get confused, causing the color in your images to come out all wrong.
With a grey card, you can easily and quickly calibrate your camera so it knows exactly what medium grey looks like. Once your camera knows what medium gray looks like, all the other colors will fall into place perfectly.