"Olive Drab" is a hard color to nail down, arguably one of the worst. I've seen photographs of aircraft painted with the exact FS# have at least a dozen different variations on the skin after time in service.

I cannot explain all the details of why, but part of it is the "flat" nature of the paint. It is just more affected (damaged) by sun and rain than bare metal or some other paints would be.

In this case there are clearly two different green used, but look within those colors and see all the little variations?

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I guess the point is, with Olive Drab, more so than other colors, there's a large window of what is considered correct.