Colour can be classed either visually by a trained cotton classer, or by a High-Volume Instrument (HVI™). When cotton is classed visually, the classer compares the sample to a standard lint sample of known grade, provided by the United States Department of Agriculture (USDA).
The colour grading of Upland cotton, considers both major and minor differences in colour. Major colour differences occur between the five classes of ‘white’, ‘light spotted’, ‘spotted’, ‘tinged’ and ‘yellow’ stained cotton, chiefly due to increasing degrees of yellowness across the five classes. Within each of these classes, the reflectance or whiteness of the fibre, is assessed across another eight levels from ‘Good Middling’ to ‘Below Grade’. There are currently 25 official physical colour grades for Upland cotton, and five grades for below grade colour. Table 2 lists the official colour grades applied to Upland cotton.
The colour of cotton as measured by HVI™ is determined by a colorimeter and defined with the Nickerson-Hunter colour model, in terms of reflectance (Rd) and yellowness (+b).