Forty volunteers reporting consistent reading problems were divided into three groups on the basis of an assessment of degree of scotopic sensitivity. Four speeded visual processing tasks involving word matching and letter and number identification were administered to all subjects using three plastic overlays, one of a colour maximizing visual efficiency, one chosen at random and one with no colour. For subjects with high scotopic sensitivity, the use of optimal coloured overlays yielded significantly better results on some visual tasks than the other two overlays. Implications of these findings are discussed.