to my eye, Diamine Emerald is a light Avacado (which I would call a yellow-green, not a true green but that's just a personal perception) - maybe something like the Diamine ink would "dilute" without a color change. But you'd have to mix and try. What I would call a standard green (WM, Skrip, Diamine Woodland green) are all too blue and would probably make mud.
A little water may help and is perfectly acceptable to try. It's a very saturated ink so depending on how wet your pens write, it shouldn't alter the color much. Try a drop per fill, then two, etc. and see if you can come up with something that behaves more to your liking but still looks as rich.
The only problem with Penman Emerald is that once you fall in love with it, you're stuck because it's so hard to find (and absurdly expensive) and people like me that hoard it are not likely to sell it.