Does the adapter that you are using come with a menu button with settings for color correction ? As mine does. Other than that there are ways to improve your color settings on extended monitors in windows 7.
First connect your external monitor then right click a blank area of your desktop, from the box that opens select Screen resolution, The window should show 2 monitors side by side, select the extended window And right click it, from the box that opens select properties, now a window should open with various tabs on the top, select "color management" Then there is another box with a button to click color management. From the window that opens make sure that display 2 is showing in the devices window then click advanced. from the bottom of that window you should see an option "Calibrate display" Then follow the on screen instructions.
The notes below I copied from the windows help file. Which you can find by clicking Start/help & support and typing in "display color calibration" this will guide you through the process
Calibrate your display
What is Display Color Calibration?
Notes
- If you have a display calibration device that came with other software, consider using the color measurement device with the accompanying software instead of Display Color Calibration. Using the calibration device with the accompanying calibration software that often comes with it can help you get the best color on your display. In general, using a color measurement instrument to calibrate your display will result in a better calibration compared to the results of doing a visual calibration (which is done in Display Color Calibration).
Hope this helps
Roy.