The role of colorant in polymer processing was investigated with respect to pressure and infrared (IR) sensing. Polystyrene was combined with blue, black, and purple color additives using twin-screw extrusion. Injection molding was then conducted using these three materials with a mold instrumented with a suite of commercial sensors as well as a custom multivariate sensor (MVS) capable of sensing melt temperature, mold temperature, melt pressure, melt velocity, and melt viscosity. Melt pressure and melt temperature are, respectively, obtained through the incorporation of a piezoceramic element and IR thermopile within the sensor head. Melt velocity was derived from the initial response of the melt temperature as the melt flows across the sensor's lens. The apparent melt viscosity was then derived based on the melt velocity and the time derivative of the increasing melt pressure given the cavity thickness. The accuracy of the temperature, pressure, velocity, and viscosity results were evaluated. Results indicated that the velocity estimates obtained with the commercial sensors and MVS had a coefficient of determination, R-2, of about 0.99 regardless of colorant. The temperature measurements and viscosity estimates were similarly and correctly found to be invariant of the colorant blend. (C) 2015 Society of Plastics Engineers