Polymer extrusion is a very common technique used for polymer processing. It applies melting and shear to processes the polymer at the molten state. At these operational conditions polymer degradation is bound to occur. Normally the degradation generates chromophores groups that tint the polymer with the typical yellow color of a degraded material. In this work we will present the development and uses of a new in-line colorimeter which was fitted at the extruder die exit to follow in real time changes in the color of the molten extruded. The optical detector is set at a slit-die with transparent windows allowing the light passes through the molten polymer while being extruded. The illumination is done consecutively lighting three colored LEDs red, blue and green, and measuring changes in the transmitted light intensity by a LDR photocell. The software, written in LabView, controls the LEDs lightning sequence, get the photocell response and calculates the RGB number and yellowness index. In this paper we will show the bench validation of the detector, using dye colored water solutions and its application during the melt extrusion of polypropylene at various processing conditions. The data is corroborated using a commercial colorimeter.