Fiber length is one of the essential and tricky-to-get characteristics of colored cotton fibers, which affects the spinnability and quality of blending yarns. Based on the optical theory and structure traits of fiber assemblies, an optical method named Random-Beard Image Method (RBIM) was proposed for measuring linear density distribution, fibrogram, of dyed and natural colored cotton fiber beards. This method involves a new two-level prediction model of reflectance and an optical algorithm for beard bulkiness. Fibrograms of 22 different cotton samples in total were tested to verify such algorithms and compared with those by the international standard method, Advanced Fiber Information System (AFIS). The average error between RBIM and AFIS was turned to be lower by 3.22%, and an excellent agreement of fibrograms between the two methods was also confirmed by Bland-Altman analysis. Meanwhile, an exceptionally high linear correlation on the average weight length of these two methods was achieved with insignificant deviation under a significance level of a = 0.01 with the T-test.