There are a few things you can do to make both monitors look the same:
1. Adjust the resolution and/or refresh rate of both monitors to be identical.
2. If one monitor is an LCD and the other is a CRT, adjust the brightness and contrast settings on the LCD until it more closely resembles the CRT.
3. Use a program like Monitor Calibration Wizard to create custom profiles for each monitor that will make them appear more similar to each other.