Canon Rebel T2i Quick Start Manual page 54

Custom White Balance
You may run into difficulties in a "mixed"
lighting scenario, for example where there is
a mixture of daylight and fluorescent or
tungsten light. If none of the presets work,
then perform a manual "custom" white balance.
Take a photo of a white piece of paper
in the same lighting as your subject.
Then go to Menu/ Custom White Balance.