High uniformity of appearance is essential for textiles. Unintended variations of colour between and within fabrics pose serious problems: their avoidance makes great demands on manufacturing and quality control at all stages from the extrusion of synthetic fibres and yarns up to the final finishing of a fabric. Most fabric inspection for quality control is visual. Unfortunately in some cases, the appeaance of a colour defect is a very poor guide to the true nature of the defect, a fact not generally recognized by the industry. This paper describes studies of the responses of observers to well characterized model colour defects on fabrics using standard psychophysical methods. The results from this approach will help the industry to understand inspection problems and provide a basis for the design of sensor systems to be used in automated fabric inspection.