This paper aims to review the problem of colour in textile effluents, the different classes of dyes available and their contribution to the problem. Through new regulations, pressure is being placed on water companies all over the world to reduce the amount of colour in sewage effluent. Dyes exhibit low toxicity to mammals and aquatic organisms and therefore colour consents are normally applied for aesthetic and industrial reasons rather than for prevention of toxicity. The absorbance, ADMI values and concentrations of dyes in effluent are examined here with particular reference to reactive azo dyes used in cotton processing. Colour consents, the problem of colour in textile wastewaters and the importance for research in this area are also discussed. Dye concentrations of 0.01 g dm(-3) up to 0.25 g dm(-3) have been cited as being present in dyehouse effluent, depending on the dyes and processes used. ADMI values ranged from 50 to 3890 units for the dyeing of cotton. It was concluded that 1500 ADMI units was a reasonable value to aim for when simulating coloured effluents. Simulated textile effluents may be used for research purposes. These should resemble real wastes as closely as possible, but it is often difficult to replicate the ADMI values, absorbance and spectra of real effluents. The concentrations of dye used in simulated effluents examined in literature varied from 0.01 g dm(-3) to 7 g dm(-3). As absorbance and ADMI values change with the types of dye used, it is difficult to relate these values to dye concentrations. A concentration of 0.18 g dm(-3) of a Red or Yellow dye or 0.43 g dm(-3) of a blue dye would provide an ADMI of approximately 1500 units and fits within the range of dye concentrations presented in literature. A dye mixture simulating colour in a real textile effluent is suggested and some limitations of simulating actual wastewaters discussed.