Color problem

Hi,

I'm currently using a Macbook Pro. I bought it about a month ago or so. Sometimes when I leave my computer screen on, the colors on the display change. Example: you know how it's gray for the Menu up top? And for Safari? When I wake my computer, it changes to blue gray. What's wrong? Is it a defect? The color goes back to normal when I close the notebook and reopen it again.