Colour-colour and colour-magnitude diagrams. Shown are g − r vs. z − W1 colour-colour diagrams (upper panel) and W2 vs. W1 − W2 colour-magnitude diagrams (bottom panel) for objects from the spectroscopic data set of the three spectroscopic classes such as stars (left column), galaxies (middle column) and quasars (right column). The purple contours correspond to the 68-th percentile of the problematic Gaussian components of the IGMM that are not dominated by objects of just one spectroscopic class. The green filled circles correspond to the means μ_k of these components. The grey scale indicates the number of sources in each diagram.