The Four-Color Method was developed to improve the accuracy of tristimulus colorimeters for measurements of color displays. It was verified that the method works well for cathode ray tubes (CRTs). The accuracy of this method has been studied further for spectral variations of displays including CRTs, liquid crystal displays (LCDs) and organic light emitting diode (OLED) displays. A theory for additional photometric (Y) correction is also given. A simulation was conducted using data of actual tristimulus colorimeters and displays having variations in their spectral power distributions. The results show that, if the correction matrix is made with one type of display and other types of display are measured, the error correction will not be effective. If the calibration and measurement is limited to one type of display, the residual errors are found to be within 0.002 in chromaticity x, Y and a color difference of 2 ΔE_ab. An experiment was also conducted to verify the Four-Color Method for measurements of an LCD. The errors of a colorimeter for 14 colors were reduced to within 0.002 in x, Y and 1 ΔE_ab. It has been experimentally verified that the method works well for LCDs as well.