Ways to achieve this dark yellow/green colour?

When you're mixing dyes, it's exactly like mixing paint. So for a green like the second one you need to make it less of a fresh green and making it more muddy. For that you'd add some of the opposite colour. In this case that's red, but I think an orange would work better. You'd just want a teensy tiny bit, too much and you'd get a mucky brown.

The first one, basically the same thing, but it's a much fresher colour it's not as muddy as the first. So I'd add yellow and and even tinier bit of orange.

You'd want to start off with a mid green, maybe directions apple green. Nothing too blue as that's taking the colour in the wrong direction. Olive greens have a yellowy base. Something like directions spring green might actually be good for the first one as its yellow based.

this isn't going to be helpful to you-- but if you try this, i'd LOVE hear what you ended up mixing (and i'd love to see pictures!).  i really like this color and am curious how it'll work out for you!

I take back Apple Green to start with. The colours looked a LOT darker on my iPad! Spring green would be best I think.

I totally agree; I was thinking start with a bright, true green and gradually add yellow and drops of orange until you get the colour you're after.