Parameter distribution and photometric indicators for metallicities. Left: distribution of [Fe/H] for the training sample composed of 4299 stars from J-PLUS × SEGUE. Right: relative importance of the features for the estimation of metallicity. The most important feature as [Fe/H] indicator is the combination (J0395−J0430)−(J0660−z) based on narrow-band filters containing the spectral features Ca II H & K, G-band, and Hα and the broad-band filter z.