NIR (top) and optical-NIR (bottom) colours for the three classes of objects considered in this work, using Euclid filters. Black solid lines display quasars in the redshift domains captured by the filters, with redshifts indicated directly on the lines. Blue lines display galaxies at (1 ≤ z ≤ 2), susceptible to contaminating the quasar samples. MLT: stars M, triangles L, and diamonds T. For the M, L ,and T, the filled are z - Y_E and hollow are I_E - Y_E.