Keep in mind that the color of the pony may affect the outcomeof the dye. Green + Orange = Brownish.. Same for purple or pink.. If you were to dye it green, I would use one that is blue, for a deep forest green, or yellow for a more vibrant green, or white for just a green green.
One other thing to keep in mind is that dye is not forever. It fades. It discolors. It bleeds through paint. It is a temporary thing. This is based on my own experiences with ponies that I dyed like 6 years ago. Other people have different opinions. A lot of people share this point of view.
Ultimately, you have to go with what you feel comfortable with. If you do dye one, you should let it set for a month or two to let the dye settle, or it will bleed through the paint. If you sell it, you may want to add a disclaimer that dye has been known to fade on ponies, but that it can last 4 - 5 years or so before beginning to discolor.
Best of luck to you on your experiments with dye!