Evolution of the sinusoidal fit amplitude for RV time series computed using spectral lines in different contrast regimes, for a photosphere at 3300 Κ and different spot temperatures. The sign of the amplitude gives the phase of the signal (Negative ⇒ anti-contrast phase, Positive ⇒ regular contrast phase). Contrast values are computed as the mean of per-line contrasts within each regime, with standard deviations as uncertainties. For each temperature, a linear fit in log(contrast) space is used to determine the effective neutral contrast C_n, namely, the contrast at which the signal amplitude is expected to vanish. Region corresponding to time series exhibiting an anti-contrast phase, shown in yellow. Region corresponding to time series with a regular-contrast phase, shown in blue. Areas indicating regions where the time series should not appear if the spectra from the spot and the photosphere were perfectly modeled, shown in gray.