MacOS – Display problem after installing Win 7 on Macbook Pro 15 (2011)

This is almost certainly a hardware problem. To determine whether it's the screen or the graphics card/motherboard you can plug in an external monitor. If you get the same problem on the external monitor then it's likely a fault with the graphics card/motherboard, if you don't get the same issue on the external screen then there's a problem with your MacBook's display.

Since it's an early 2011 model it should still be in warranty. I'd bring it to an Apple store and have it checked out for repair or replacement. On the plus side, you may get a brand new MacBook out of it.

This could be related to the PRAM which stores startup disk and monitor depth settings.

Resetting PRAM and NVRAM

1. Shut down the computer.

2. Locate the following keys on the keyboard: ⌘, ⌥, P, and R. You will need to hold these keys down simultaneously in step 4.

3. Turn on the computer.
4. Press and hold the ⌘-⌥-P-R keys. You must press this key combination before the gray screen appears.
5. Hold the keys down until the computer restarts and you hear the startup sound for the second time.
6. Release the keys.