I think Skuzzy is saying that if all the bare metal panels are made of the same material, i.e. aluminium, then they should be the same base shade on the spec, power and environment maps, excluding weathering effects. I'd add that if there are some metal panels made of heat proof metal or armour plate then these might have a different finish and be shaded differently on the maps. Making individual aluminium panels darker or lighter on these maps so they reflect light differently to each other would help the illusion of the skin being made of a patchwork of metal panels but doing this is likely to get your skin rejected. Making the panels different colours on the diffuse map is OK because they plainly were different shades in RL (different gauge and alloys etc).