I think the issue is there are a lot of slightly different shades of olive.
Here is what Wikipedia says:
Olive is a kind of muddy green color. In actuality, it is really a shade of dark yellow (when gray or black is added to yellow, the various shades of the color olive are produced). Some dark shades of olive can also be made by mixing a darker color (like brown) with green.
The color of old U.S. Army field uniforms is a shade of olive called olive drab (shown below) that was used because it was hard to see against trees and grass.
Very good Proof of The Olive Colour.
Even though I've eaten my share of Olives over The Years The Colour variation,as with all The other Colours many of which we all have,goes from A Black Olive to a very light Olive Colour.
I'm not over keen on Martinis I prefer Beer Wines & a Wee Drop of The Good Spirit.
Another thing over The years I've bought heaps of Olive Pheasant Rump Feathers for Mrs Simpson's & other Flies very seldom have The Colours been an exact match.
Olive has got to be the most confusing noun/adjective as related to fly tying colors! Most of us automatically think of an earthy green to be the color of olive, but if you consider the shades that the olive fruit can be the variations is numerous, anywhere from tan to green to purple to black. Add in the shades of olive that the Irish have for fly tying materials and it get more confusing. (light olive, medium olive, dark olive,green olive, sooty olive, dirty olive, black olive, golden olive, copper olive, brown olive, Donegal olive, Ballinderry olive, orange olive and then the mayfly shades might add another dozen)
As there are no real standards the colors from one company to another can be different, I have accepted the idea that the name of a color is usually just an indication and not an exact science.
Have you ever wondered why the adams is the most popular dry fly pattern?
I think it is because the body of the fly is gray. Gray is a combination of white and black. We think of white is a color, but really it the combination of the entire visible wave lengths. White is a reflection all wavelengths. Black is the absence of any reflection so it is not a true color. Black is the absence of a color and white is the presence of all visible colors.
Why then is the most popular dry fly gray instead of olive, brown, tan, yellow or any of the other mayfly colors? How many common mayflies are actually gray in color?
I think the adams imitates many mayflies because color is the least important of the 4 features of a fly. AND the underside of a floating fly is shaded. I think gray can mimic the color saturation of the mayfly color the fish see, especially the olive and browns. So if you don't have the right "shade" of olive, there are fish that will be fooled by the same "shade" of grey.
How about chartreuse? That's another shade of color that there seems to be much disagreement about.
I agree with flytire's assessment, except that same reasoning can be applied to most anything that fish eat. Match the hatch! I stopped trying to match specific shades of colors a long time ago. I found as Silver Creek stated, that most of the time, exact shades or even specific colors made no difference to the fish. I've seen fish get picky, and not just trout, but cannot say with 100 percent certainty it was necessarily due to color.
I've gotten to the point, that when I select a particular color, such as olive, I end up with various materials that cover the range of shades, which could be black olive, to brown olive to the lightest pale green. So, I tie with what I like the appearance of, and many flies of the various shades, ( such as Woolly Buggers, you can't have enough of them) so I have it all covered.
I'm also not a big fan of humogeneous color when it comes to materials such a dubbing. I've yet to see any insect or other critter that fish eat, upon close inspection, that was a single, homogeneous color. I much prefer to mix & match, which IMO looks more natural.
If you're dyeing your own materials, try dyeing a white feather & a grizzly feather the same color. You may get the same color, but the grizzly feather will appear to be a different shade. At least it looks that way to me. If you dye over white, or over something like a natural red/ brown hackle, you get two different shades. The possibilities are endless what you might end up with using a single color dye, such as olive, but different natural colors of materials. (hackle, hair, etc.)
So, stop worrying so much about it, pick a few shades you like,and tie with it. You'll catch fish with whatever you choose!
When the fish get picky, which is not usually too often, I can usually show them something they'll eat.