W1 − W2 versus W2 − W3 colour-colour diagram of spectroscopically identified quasars in this work, where Gaia-detected sources are shown as blue open squares, and sources not in Gaia DR3 are shown as orange circles. The density distribution of the full identified quasar sample is indicated with green contour lines. Euclid point-like sources are shown as grey triangles. To ensure the reliability of the colours, only sources with adequate S/N (w1snr > 5, w2snr > 5, and w3snr > 3) are shown.