IIRC the pannel around the exhaust on the P-51 was stainles steel and that is why it will diss-color differently to the allu cowls.
Alclad sheetmetal has a grain direction as it comes off the mill. The rule of thumb is that you want to bend metal accross the grain, therefore you might have bent pannels with the grain running in different direction to others.
This grain plays with the light falling on it if you look at it from different angles, that is why pollished planed look a whole lot more "uniform" in texture.
Remember that when allu is in short supply they sometimes used magnesium alloys aswell.
It is also true that different alloys are used to make different hardness of alclad sheets, this will also have a different look to the sheet.
When alclad weathers it becomes slightly dull and gets a grey look to it.
Here is my 1/32 J-6 . I used only dark alluminium and airframe alluminium over a gloss black base to give it some depth. I am also a cheap skate and don't buy many different alclads. The more coats you put on the lighter it gets. I sometimes put on a few coats of say airframe allu, then mask of some pannels and shoot some more of the same alclad on, this gives a nice tone difference yet is subtle.
Protect the alclad after decals are on with ONLY alclad's range of clearcoats, they do not take away from the alclad look.
Hope it helps