Caucci and Nastasi made an observation that no color in nature is pure. Rather, if you look closely enough, the color we see is comprised of a combination of other colors. Following that observation, they blended different colors out of a range of (I think, 4) major colors.

I found a distributor who sells a "full kit" of their 16 "Match the Hatch Colors".

This is an example of the Pale Olive and the specific insects it imitates in color: