These parts were injection molded on the same machine, on the same day, from the same lot of material (a toughened, glass reinforced nylon 6/6, with a custom color). The color variations were caused by poor dispersion of the color concentrate. The concentrate was collecting at the bottom of the shipping container, due to the higher density of the metallic pigment.
Once the problem was identified, a mixing protocol was developed, where each lot was tumbled prior to processing. The color concentrate then became uniformly distributed, allowing for a consistent molded-in color.