I don't think an egg coloring kit will cut it since the dye is probably food coloring based. Rit dye for synthetics would probably be a better choice.
I'm pretty sure
@MonsterIceCream was experimenting with dyeing ponies?
I've been summoned, sorry for the late reply! My immediate answer is 'you don't know until you try', but my gut is telling me that Easter egg dye sets are probably not strong enough to penetrate into pony plastic deeply enough to become permanent. You may be able to use them to some extent with white nylon hair, but I do worry they might fade into nothingness over time. My concern would be that the dye would fade and look more like stains instead of a nice color.
For a galaxy style dye job, I would personally recommend Rit Dyemore for synthetics (not just 'Rit Dye', it has to have the Dyemore in the title), but I've also heard that iPoly synthetic dye on Amazon works for vinyl also. I would suggest taking off the head, then dip-dying the body into a boiling pot of dye from various angles, much like dip-dying an Easter egg, to achieve the galaxy type effect. You could also experiment with soaking cotton balls or rags in dye, and laying them over the pony in patterns. The dye will take longer to set if you're not applying heat as well, so I recommend leaving them overnight. (Alternatively, heat gun?)
I have experimented with hot glue as a dye resist, and it worked extremely well; however, the heat from my dye boiling re-melted the hot glue, and it spread onto other ponies when it melted onto my tongs, as well as gummed up my dye pot. I would suggest boiling the dye, then pouring it off into a heat-safe container before you add a pony into it, or simply try it with a lukewarm or even cold dye pot. I have not tried it with the masking fluid listed above, but that's going to be a future experiment! I would love to find an easy, simple dye masker for ponies.
Best of luck to you!
