A visual search experiment was performed to examine whether the detection of constant chromatic relationships occurs in parallel. We used 6 stimulus conditions, corresponding to different combinations of three variables, relevant to description of displays made of different coloured regions: (i) the number of items; (ii) the number of background colour ratios; (iii) the total number of colour ratios. An illumination change was simulated in all trials. In half the trials, the simultaneous; reflectance change of one item was also simulated. When the number of colour ratios is constant, detection becomes worse as the number of items increases, as predicted by serial processing of reflectance changes. However, background articulation favors relational colour constancy, by providing information about illumination changes. (C) 2000 John Wiley & Sons, Inc.