Colors differ in the dual monitor
Check this out Color Calibration 101 (codinghorror), it should help! But I would recommend to buy a display calibration device (absolutely useful when using dual display)! The good ones are pretty expensive, you can share it with friends to afford the price.
A large part of the problem will be that the LCD panels for external displays and laptops won't be manufactured in the same way in the same factory so there will always be differences in the colour profile of the screens.
You can get devices that help you to match the colour profiles of monitors so that what you see on one is the same as on the other, but they tend to cost a bit.