Blending of thermoplastic polyester with fiber-forming polyamide in the production of melt-colored melt-spun fibers results in improved color strength and aesthetics, and dimensional stability. The instrument includes a source of polarized light to illuminate the sample, and a detector for measuring the light reflected by the sample. The IO window has a fixing slot and a first screw hole. The first container is also rupturable to dispense the mixture therefrom.