For natural metal aluminum I used the FS call out for Silver, which is HEX 888888 or RBG of 136, 136, 136.
This is a medium gray color than need an Alpha channel to give it the shine of natural metal. It is good to experiment with the Alpha Channel intensity. I like to keep it low, because a high intensity will give sharp angles in Flight Sims with the environment bitmap. Thus, I keep the gray in my Alpha Channels at RBG 175, 175, 175.
Note the fuselage of my repaint of Aeroplane Heaven's F3F-2 and compare it to the wings. Both use a base color of the silver above, the fuselage has an Alpha of 175, the wings an Alpha of 196, as the aluminum was doped paint on the wings.
Caz