The distribution of NUV − r (top), NUV − J (top middle), u − J (middle), g − r (bottom middle), and u − r (bottom) for galaxies classified as discs (blue histogram) and smooth (red histogram). The completeness (solid lines) and purity (dashed lines) are shown for selecting early-type (red lines) and late-types (blue lines) galaxy samples. These figures show how changing the colour threshold to morphologically classify galaxies based on their colour results in incomplete samples. The black dashed vertical line shows the colour threshold at which a compromise between purity and completeness is achieved (i.e. they are equal). If there is no other option but to use colour to make a morphological classification, then the purity and completeness are both maximized when a threshold of NUV − r = 4.96, NUV − J = 7.09, u − J = 4.55, g − r = 0.74, or u − r = 2.34 is employed.