By studying the minimum legible font size of different colors for middle-aged and elderly users in mobile digital reading, this research aims to provide a reference standard for the age-appropriate design of mobile applications. Through experiments, this research analyzes the minimum legible font size in the four typical gray-scale colors in mainstream information apps—weak black (1F1F1F), dark gray (525252), gray (858585), and light gray (B8B8B8) respectively when 95% or more middle-aged and elderly users aged 50-70 can identify. For middle-aged and elderly users, the minimum legible font sizes for weak black, dark gray, gray, and light gray in digital reading on the mobile devices are 7.17 pt, 7.17 pt, 8.19 pt, 9.21 pt respectively, among which the light gray font is significantly more difficult to recognize compared to the other three colors. Designers and developers should be cautious in using light gray and other lighter colors in mobile applications for middle-aged and elderly users. The minimum legible font size of weak black and dark gray are both 7.17 pt, which can be set as the minimum font size threshold for the auxiliary information in mobile digital reading (such as system settings, function buttons and labels).