Visual search for colour targets that are or are not linearly separable from distractors

D'Zmura [(1991) Vision Research, 31, 951-966] reported qualitative differences in visual search rates for a target colour in a background of differently coloured distractors depending on their colour configuration in CIE(x,y) space. A target colour that was chromatically mid-way between the distractor colours resulted in steep search slopes. A target off the distractor-distractor line, "popped out". We replicated his finding in several loci, investigated several potential confounds, and discovered boundary conditions for the phenomenon: for a given target, the effect of collinearity dissipates with increasing distractor-distractor colour difference. Furthermore, within limits, performance was dependent on the target to distractor-line distance.