Effectiveness of the f415 photometric filter. Panel a: I vs. C_BVI CMD, as in Fig. 3a. Stars are colored azure and red according to the classification described in Sect. 3, i.e., canonical and anomalous. Gray errorbars represent the average photometric uncertainties determined in different magnitude bins. Panels b and c: externally calibrated Gaia XP spectra of two couples of reference canonical and anomalous stars, indicated with pentagons and triangles in panel a. The gray shaded rectangular regions indicate the f415²⁵ filter, designed to identify canonical and anomalous stars. Fluxes are expresses in units of Wm⁻² nm⁻¹. Azure and red shaded regions indicate flux uncertainties. Panel d: m_f415²⁵ vs. m_f415²⁵ − I, where m_f415²⁵ is the magnitude computed by convolving externally calibrated spectra with the f415²⁵ filter shown in panel a as in Eq. (5). Reference canonical and anomalous stars are enclosed by empty pentagons and triangles, while metal-poor and metal-rich stars from Tautvaišienė et al. (2022) are marked with empty circles.