I think the Descent 3 approach of building robots based on purpose works well. Security robots, miner robots, utility robots, and so on. Within each class, there were similarities. Tubbs, Gyro, and Orbot had very different models, but their texturing was consistent with the security theme - blue, gray, and spinning emergency lamps.
Descent 2, while lacking the explicit robot classes, tended to have robots that complemented the level's scheme. Blue, brown, and green robots in Quartzon, icy blue and white robots in Limefrost Spiral.
Either way, err on the side of muted colors. These are meant to be dirty mining machines, after all. The PTMC is not responsible for putting a fresh coat of firehouse red on every new unit.
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