I am not sure what you mean by calibrate, so ill take a few stabs.
- You can try pressing "Auto" on your monitor, if its an LCD.
- YOu can try installing f.lux That will manage your color tint according to the time, and brightness setting.
- If none of the above are answers to your question, then try adding a few more detials to your question.
EDIT: after OP added a detail, this should help: type this on a console/terminal.
first, just type
xgamma to get the RGB values, in case you want to revert. Then,
xgamma -gamma 0.9 the 0.9 is the gamma value. Try a few diff combinations of RGB.