Distribution of the mid-infrared W1 − W2 color (3.4 μm–4.6 μm, Vega) of quasar candidates in our KiDS sample, derived from a cross-match with all-sky WISE data. We show histograms for all KiDS QSOs matched with WISE, as well as for two examples of the probability cut: p_QSO >  0.7, which is recommended to increase the purity of the sample, and p_QSO >  0.9 to illustrate how the resulting W1 − W2 changes when the minimum probability considerably increases.