I've never had troubles getting bright greens. I think it's just a question of dye concentration, and of course the temperature problem we've often seen with turquoise. I always use a LOT of turquoise, at least 4 teaspoons per cup, maybe more, or half that much cerulean, and then add from 1/4 teaspoon to 2 teaspoons of yellow MX-8G. For a bright lime green you'd use the larger amount of yellow. Here's a picture of a bright green LWI rayon outfit I dyed with turquoise MX-G, yellow MX-8G, and a navy that I like which is not available from most retail dye suppliers, reactive blue 9. Not sure if I also used some blue MX-R, or if that's just the blue number 9.

I was aiming at a much bluer green, but my Dharma turquoise had gotten very weak with age. Usually inadequately bright turquoise is due to low room temperature, but in this case the temperature was fine, and the dye was old. (I now prefer other suppliers, but this wasn't really their fault, in that it was a couple of years old, and they claim a shelf life of only one year. My reactive blue 9 is much older but still going strong!) This color is too bright for me or for the friend I made it for to actually wear, but it's so pretty that I still haven't gotten around to overdyeing it to be less blinding and more wearable.
Jacquard's
MX color mixing chart has a number of greens, including yellow green, chartreuse, vermillion green, lime green, spring green, and shamrock. Their lime green is 2.75 teaspoons of yellow MX-8G to 1.125 teaspoons of turquoise MX-G. They don't give the amount of water this is to be dissolved in, presumably because this is a chart for solid-color dyeing, but I still find it useful now and then.
Paula