Unobscured QSO intrinsic colours, as a function of redshift (ζ), for the bands i and z. The functions C_i,z(ζ,U_min), are shown for the different values of U_min, using redder traces for larger U_min, and greener for smaller U_min. Their weighted mean C_i,z(ζ) is shown in black. The Lyα line affects the i band photometry for ζ> 4.62, but we restrict our analysis to ζ< 3.35 to have enough QSOs per redshift interval. For ζ> 3.35 the estimated C_i,z(ζ) becomes noisy.