Achieving a stimulus-responsive, multi-colour, long-lived luminescence based on single-emissive-center carbon dots (CDs) is highly desirable for numerous promising applications. However, the fabrication of such materials remains a formidable challenge because of the limited paths of exciton transfer. Herein, we report a facile strategy to achieve a colour-tuneable afterglow by selectively activating different exciton transfer channels in a carbon dot-cyanuric acid (CA) composite (CD@CA) by alkali induction. Upon alkali treatment, CD@CA exhibited a noticeable afterglow colour change from cyan to yellow with an excellent reversible pH response. Remarkably, the yellow afterglow efficiency is as high as 34.8% and its average lifetime can reach 0.535 s. Detailed analyses revealed the existence of two exciton transport pathways within the system. Excitons can be transduced from CA to CDs via Förster resonant energy transfer, leading to a cyan afterglow. In an alkaline environment, this channel was destroyed, and the inherent phosphorescence exciton transport channel of CDs was simultaneously activated and enhanced. Alkali-induced CD ionization and the formation of rigid crystal networks boost intersystem crossing rates and reduce non-radiative transitions, resulting in a bright yellow afterglow. Furthermore, based on the colour-tuneable afterglow properties of the system, we illustrated the potential applications of CD@CA in advanced information encryption. This study provides guidance in the development of multi-colour afterglow materials with stimulus-responsive characteristics to meet the growing demand for highly secure information storage materials.