Difference in colour between a solar metallicity SSP burst at z = 3 and two SSP bursts at z = 1.5 when viewed at z < 1.4. The two younger SSPs have metallicities of approximately 0.25 and 1 times solar. These models illustrate the most extreme colour differences likely to occur in a quenched system with two different stellar populations. The oldest burst represents a bulge population and the youngest a disc. In reality both are likely to consist of stars from more extended periods of star formation, decreasing colour differences between them relative to those derived here. Nevertheless, the models demonstrate that the colour gradients observed in the CRSGs studied here can plausibly be explained by a mix of age and metallicity related gradients, with the age-induced gradients dominating at earlier times over the metallicity gradients.