I have two projects coming up that are British, an airplane and a tank (Valentine). The two share colors, for example, middlestone. Am I safe buying one color for both projects, or was there a notable difference between the aircraft and armour colors?

You could try lightening, or darkening, the color for one of the vehicles/aircraft, so that either color is neither right nor wrong, but reflect a normal variation between paints.

Are you sure that Middle Stone was used on vehicles in the timeframe you are depicting? I'm on the train so don't have any references with me but Mike Starmer mentions it being in only limited use:

http://www.mafva.net/other%20pages/starmer%20camo.htm

He has previously posted mixes on ML for the majority of ME colours and his books are well worth getting too, as they contain paint chips matched to original standards.