Colorful transitions of various acid-base indicators have been used to illustrate the concept of chemical equilibria, especially in spectrophotometric determination of the equilibrium constants or pKa values. To accommodate an online laboratory environment necessitated by the COVID-19 pandemic, this work employs color analyses from digital images to determine the p"K"[subscript a] values of a double-range indicator thymol blue. The experiment has been developed into an online activity to teach chemical equilibria and dissociation of a diprotic acid. The p"K"[subscript a1] and p"K"[subscript a2] values determined by students are in good agreement with the literature values at 1.60 ± 0.05 and 8.70 ± 0.06, respectively. The results demonstrate a powerful application of RGB color analyses in determination of the p"K"[subscript a] values of thymol blue without access to spectrophotometers.