Graphical representation of Eq. (3) for the parameters of interest assuming a stellar photospheric temperature of T_ph = 5000 K and a black-body for the spectral energy distribution of each surface element. (a) Photometric amplitude as a function of the modulating filling factor for different spot temperature contrasts. A non-modulating filling factor δ₀ = 0.05 is employed, but adopting other values has negligible effect at this scale. The gray area covers the spectral range from 400 nm to 1 μm. (b) Difference in activity-induced amplitude for photometric bands BV R with respect to the I band as a function of spot temperature contrast. A value of 0.05 is adopted as modulating filling factor (δ_M) and examples for non-modulating filling factors of δ₀ = 0 and 0.1 are shown. The gray lines illustrate how the bi-valuate nature of the temperature contrast effect can be resolved if more than two bands are used. (c) Difference in activity-induced amplitude for photometric bands BV R with respect to the I band as a function of the non-modulating filling factor. A modulating filling factor δ_M = 0.05 and a spot temperature contrast ΔT_sp = 600 K are adopted.