Although it is widely known that the foundation color gradually changes over time after applying the makeup, no scientific clues are available, thereby remaining the "darkening" phenomenon equivocal. This study measured the discoloration of liquid foundations and suggested indices to describe the discoloration over time. Four liquid foundations were applied to opacity charts with a thickness of 100 mu m, the surface color was obtained using a non-contact spectrophotometer, and reported in the CIE 1976L*a*b* color space. A long-term measurement for 24 hours was conducted in that the initial 2 hours were measured with a 60-second interval. The discoloration was observed as the decrease of both lightness and hue angle (h* of CIEL*C*h*) of liquid foundation over time. Different degrees and speed of color change were observed across the four foundations. Also, the discoloration was tested on three healthy human skins in 2-hour intervals in repeated measurements. Positive correlations in changes of lightness and hue were found among the opacity charts and human skin. Furthermore, the study suggests indices, "Dt" and "T Delta E" to describe the quantitative amount of discoloration over time and time duration of discoloration, respectively.