Flux density variability as a function of spectral index variability across all epochs for both the Arches and Quintuplet clusters. The black dotted lines indicate ∆S/σ = 10 and Ξ = 10 values, which are indicative of high flux and spectral index variability. Table 4 shows the uncertainties of the plotted values, but we refrained from showing error bars in this plot for the sake of clarity.