A light irradiation method was found to promote coloring in sweet pepper fruit (Capsicum annuum L.) harvested at the breaker stage of mature green fruit. In summer and autumn culture, heating systems are not usually used, and a large amount of uncolored fruit remains after harvest because of low temperatures and/or insufficient sunlight. We investigated the use of light irradiation to enhance the color of the fruit post-harvest and found that light intensities between 50 μmol·m⁻²·s⁻¹ and 200 μmol·m⁻²·s⁻¹ made no difference to the coloring rate; however, higher intensities resulted in an increase in the carotenoid content, which is responsible for the color in red sweet pepper fruit. Although temperatures of 15–25°C with light irradiation are considered to be appropriate for fruit coloring, the transpiration rate was found to increase in propotion to temperature rises, and the fruit wilted at 25°C. We also confirmed that fruit colored more than 50% by light irradiation continued getting colored in the dark at temperatures above 15°C. This combination of light irradiation and dark processing may potentially improve the ripening process efficiency and preserve the market value of fruit.