The objective of this research is to further investigate factors that may have significant influences on cotton color measurement but are not considered in the current cotton color grading system. These factors include the redness content (a) in cotton chroma, and the presence of spots and trash particles in cotton. The study was based on the color data of the USDA physical standards for U.S. upland cotton and a number of selected samples measured by the imaging colorimeter (CTC) developed from the previous research and a Minolta CR-210 colorimeter. Three major results are found through this study: (1) the a content makes up 10% to 33% of the chroma, varying primarily with the major color category (white, light spotted, spotted, tinged and yellow stained). Within the same category, a is less variable than yellowness b. An approximate +a range for each major color category was determined. (2) CTC is less sensitive to the presence of spots and trash particles in the sample than CR-210 because CTC has a much larger viewing area. (3) The influence of spot and trash on the cotton color measurements depends on their sizes and colors in the sample. A change in cotton color data made by spots and trash may lead to a change in color grade. The computational removal of these regions from the sample image in CTC is effective in minimizing the effects of spot and trash. In addition, the paper introduces a new color diagram built on the measurements of reflectance-redness (Rd~a), which identifies the useful role of a in cotton color grading.