||This paper aims to determine the effect of the smartphone warm color temperature functionthat relieves display’s HEVL (high-energy visible light and short wavelength series blue light), which isknown to cause suppression of melatonin secretion on actual image reproduction quality. For this study,the author of this paper measured the display based on the color difference in 26 sampling colors. It was foundthat for correlated color temperature (CCT) of 4000 K or less, the color difference rose sharply, centeringaround red and green. In hardware or software, a low CCT was realized by reducing the output centered onblue and green, but in actual color quality, a problem arose in the red and green channels. As far as tonegradation is concerned,1E increased for CCT of 4500 K or less while the accuracy of the shadow detail wasreduced. With regard to color gamut reproduction, for the coverage of sRGB color space, the color gamutbecame narrow for CCT of 5500 K or less, and for volume, the color gamut became narrow sharply for CCTof 4000 K. It was found that the maximum CCT changes to prevent a decline in melatonin secretion at alevel of minimizing the degradation of image quality is 4000–4500 K.