I wouldn't know what to do with a Leela pony myself. I like all three futurama ponytars that have been posted although I don't think that's how I'd have gone about any of them. Leela is tricky in that her most typical outfit is very neutral. I guess maybe a very pale grey (paler than Bender's grey) might be what I'd go with for her coat. I'd say the pink of her lipstick/lips would be a good colour but that would make her look too girly I think...
Serpentine seems to have picked up the knack to do ponytars a lot quicker than I have as I like all of the ones she's done (especially the Unosartatar) but then I'm under the impression Serp's got a lot of experience with inkscape or something that yields similar results to inkscape whereas it's still a pretty bizarre way to draw pictures for me (I think if I had the attention span to follow tutorials I'd probably be better with it
(I've not seen enough Futurama to remember Leela wearing lime green at all especially as I enjoy the show but not enough to have bought dvds and it's shown very sporadically on the tv channels I get.
I kinda think if it were me I might make an exception with the flesh colour ponies for Futurama ones just because that sort of look fits with the series, with Oots I wouldn't give a caucasian character's pony fur the same colour as the skin though... it depends how much of which series you want really because there's quite a difference in approach between Futurama and My Little Pony so a crossover could go in a lot of ways.)