It depends on what colour you want to make her. If you want her a lighter or completely different colour (yellow or green), dying isn't going to work, and you'd have to paint her.
If she's going to be dyed darker, or a similar colour, go ahead and use dye.
For dyeing, I use Rit Dye, and a little bit goes a long way. If you want the hair a different colour, I haven't yet found a way to mask the hair, so most of the time, I rehair about three weeks after I've dyed the pony (this stops the hair from going odd colours)
For the darkest colours, I found that 30 minutes is the optimum time to leave the pony in the dye. Any shorter, and it doesn't quite go as dark as the packaging suggests (at least so far for me).
There are a lot of good tutorials on the web about dyeing and painting. Your best bet would be to read some of them.
aikarin.com
Tic-Tac-Toe Pony
Both aikarin and Tic-Tac-Toe have tutorials on painting.
I've yet to find tutorials on dyeing ponies, but if you ask about it, someone here is bound to know more.
My only advice for dyeing is to keep an eye on the pony while it's dyeing, and to experiment a little. You can mix two dyes to get a shade closer to what you'd like, but make sure you test the shade on something else first. Hope that helps
