Display color change

The display on my MacBook Pro laptop has suddenly changed so that instead of white here I get a light yellow. What has happened?

I'm not sure how this can happen all of a sudden, but I have a suggestion.
Launch System Preferences (fastest way is Apple Menu > System Preferences)
Click on Display
Click on the Color tab
Select Color LCD in the left column
If this does not reset the color, click the Calibrate... button on the left. This launches the Display Calibrator Assistant
At the step called Target White Point (see the current step name at the left of the window) make sure you select Native

Hope this helps