Among some concerns that have been limiting the widespread application of clay-based supplementary cementitious materials (SCMs) is their reddish-yellow colour. In this study, we focus on the use of L-ascorbic acid (AA) to modify the colour of clay that has already been calcined for use in limestone calcined clay cement (LC3) through a thermal activation process simulating the residual heat and aimed at transforming hematite (reddish) to magnetite (greyish). Temperatures between 80 and 600 °C were considered. Concentration of L-ascorbic acid, time, sample amount and fineness were also varied. The colour-modified clay was characterized through XRD, SEM and isothermal calorimetry to determine optimal conditions for application of the technology in the cement industry. There was no significant loss in mechanical strength in the use of colour-modified calcined clay with up to 20 wt.% ascorbic acid at 3, 7 and 28 days of curing.