Rest-frame (u − r) colour of the z > 6 contaminants and stable intermediate-z galaxies against the fiducial redshift, based on real COSMOS photometry. The z > 6 contaminants and stable intermediate-z galaxies are identified through Euclid photometry alone. For both populations, we plot the median (u − r) colour in Δz = 1.0 fiducial redshift bins, where the error bars represent the 16th and 84th percentiles. The individual data points of z > 6 contaminants are shown with small red stars.