1. A white balance method for a display image, comprising: measuring gray levels of a plurality of colors indicated according to a plurality of gray patches under different color temperatures, wherein the colors comprise a basic color and a plurality of reference colors; obtaining a plurality of reference color gains of each corresponding color temperature according to an average value of the gray levels of the reference colors of each corresponding color temperature and the average value of the gray levels of the basic color, and obtaining a ratio of each corresponding color temperature according to the reference color gains of each corresponding color temperature; dividing the display image into a plurality of blocks; obtaining a plurality of reference color gains of each block according to the average value of the gray levels of the reference colors of each block and the average value of the gray levels of the basic color, and obtaining a block ratio of each block according to the reference color gains; generating a selected number of each corresponding color temperature according to the block ratio of each block and the ratio of each corresponding color temperature; and selecting the reference color gains corresponding to one of the color temperatures and a basic color gain for executing a color compensation of the display image according to the selected number of the color temperatures.