Looking for a way to measure residence time distributions of an 80 micron fluidized cracking catalyst (FCC) powder, a simple measurement technique was discovered that quantifies tracer color concentration. Using a color spectrophotometer that measures percent reflectance as a function of wavelength, a calibration curve can be constructed for standard mixtures of dyed and un-dyed powder. This calibration curve can then be used to determine the color concentration of an unknown sample by measuring its reflectance. The effects of operating parameters such as dye strength, aperture size, surface roughness, sample volume and depth, and continuous flow were all evaluated. This spectrophotometric technique was found to be a quick and simple way to measure colored mixture concentrations. In addition to being ideal for residence time distribution applications, it has the potential to easily quantify mixing in any unit operation, batch or continuous. (C) 2015 Elsevier B.V. All rights reserved.