Great info on the P40 and Boston, Paul and Andy, nice work guys.
Keith, the numbers shown for the Olive Drab and Neutral Gray are the US colour numbers, not those of a paint manufacturer. They were both standard US colours, as used, for example, on B17's in the ETO. In 'official' use, they would both be followed by the FS number, for example, Olice Drab 42, FS !?!?.
There were two Olive Drab paints, 41 and 42, and I can never remember which was the darker of the two! However, they both faded and weathered quickly, so anything resembling a greenish brown, through brownish green to a greenish purple could be seen!