How to get Army/Olive green hair? HELP

You need the yellow back in you hair that the grey is cancelling out. If you don't, the hair will always pull grey green rather than olive. So fade the grey out.

You need to custom mix a dye. You'll need a yellow based green to start and then something to knock back the brightness of most of them. Most greens are bright or jewel toned because that kind of green can be tough to wear. I'm thinking a tiny bit of red, but I'm not 100%. Hmm. That's how I'd start if I were mixing it in paints anyway! It's pretty much the same thing.

Okay, I see. I could get rid of the grey. I looked at the color shade calculator (based on the color wheel), and it said that I should indeed add some red to it. But it also said that I should include some blue too. Do you think that's true?

Maybe. But there will be blue in the green anyway. Dyes aren't quite as straight forward as paints as they're geared to what people will wear, so even yellow toned greens are a bit more blue toned than you'd expect. I think Special Effect Limelight would be a good starting point.

It's easier to knock back bright colours than to add brightness to or adjusting a more muted colour, so I don't suggest getting a blue green and adding yellow.

It could well need a little blue, but the teeniest amount.

Yes, that makes sense. Thank you so much for your help!