I always expected that black was the easiest color, since you just add enough pigment to wipe out any colors from other materials. Are they implying that that brown color is the natural look of the materials they used, so the simplest to engineer?

From what I understood, any color and material involved in high precision manufacturing requires careful design and thorough testing. They likely prioritize the brown color and material due to branding, so changing this to anything else requires redoing large parts of the pipeline.