Luminous
Machinedramon favored dark environments; his optical sensors were very sensitive instruments, capable of turning low-light environments into bright ones and bright environments into painfully bright ones.
MetalSeadramon, on the other hand, preferred the light. Whether it was sunlight, moonlight, or starlight, it was an invaluable navigational tool for swimming, and so the presence of light always made him feel at ease.
The massive cavern that the two digimon called home was warmly lit and luminous near its front due to the sunlight that flooded in through the entrance. But gradually, it turned black as pitch as it extended deeper and deeper into the earth, and so Machinedramon tended to gravitate towards the rear of the cave, whereas MetalSeadramon stayed closer to the front.
Or rather, that was the arrangement when one of them was away from home. When they were both in the cave together, each would be found at the other's side, no matter how light or dark it was.