One bit of good news is that it "seems" like ppl have less challenges dyeing G1s as opposed to the g3s.
Dislaimer: I am not experienced much with dyeing ponies. lol. I prefer dyeing hair
With dye, you have to consider the original color of the pony in your formula. For Twilight, maybe a dash of royal blue and petal pink? Try to start by adding very little blue, you can always go darker once the color is right, and slowly.
Nightlight kinda looks like a double dose of Royal Blue, but again I'd go slowly.
Pinwheel maybe fuchia + a dab of violet?
Regrind, cancer, etc. will not be removed with dye. Make sure to rub pony down with rubbing alcohol to remove any leftover acetone. It can help to add a teaspoon or so of vinegar to the dye bath to help the dye set after the pony has been in it for several minutes. Crystal rit seems to be stronger, but there is a greater chance for it to be uneven if particles are not fully mixed. If you can, let the ponies sit for weeks or months to really let the dye cure.
Dyeing is an art form! If you can, it helps to have test baits. Have fun!
Whew!