I used Dharma procion dyes for my diapers. I've heard through many sources that procion dyes are the best quality dyes you can use and Dharma is a great place to get them (though apparently not the only one).
I then used Dharma's tye-dye recipe but I didn't actually tye-dye it per say. I used these little plastic syringes I had at home (the kind they give you in the hospital to finger feed a baby) and basically dripped the dye onto the diaper into the pattern I wanted. Then followed the same instructions as for tye-dye. Time consuming but the results are pretty cool!
One tip, think about what colors will be next to each other and how well they'll blend into each other (some bleeding is inevitable if you want your colors to touch). For example, red, yellow and orange go well together and look nice. But red and blue aren't as nice because where they bleed into each other it just ends up a dark muddy color.
Have fun! I enjoyed dying my diapers and it was so fun to see them on my baby's bum, especially in summer when I'd often let them wander coverless!