Short answer, there are different shades that people might call Olive Drab.
Longer [but not a professional] answer... Depending on time period, and where/when the aircraft was painted, different depot's may have had a different shade mixed and/or available. Also weather had various effects on OD. Some paint lines (MMP) have premixed color ranges that go from very dark to very faded and all tones in between. That said, you can easily mix up a lighter or dark shade if you need to without too much difficulty.
