Scheme of the color slope in the B- to V-band flux plane between two measurements t_i and t_j with photometric uncertainties. If Θ falls below −45° or above 135° we add or subtract 180° respectively. Restricting ourselves to the half circle, marked by the red arcs, ensures proper averaging of for low values of τ, where photometric uncertainties dominate the slopes. A restriction to the full circle, or −90° to 90°, would bias the slope towards 0° if the photometric errors are large compared to the true change of flux.