from testing, red apples increase digestion rate and indigestion by a moderate amount (and while unconfirmed it seems to have minor boosts to weight gained by preds when they digest other things like students and paper as well as other apples)
green apples decrease digestion and have a low indigestion increase meaning you have more RNG time but less chance of RNG indigestion compared to red apples
crazy apples (purple) are rare but increase indigestion by a substantial amount and add a crazy modifier to the pred, making them shake a lot and be unable to eat/keep anything down while the apple is digesting so using one when you are about to leave with the book is a good idea (but with the RNG nature you can still fail if you are in a pred's stomach with this apple as she'll probably vomit everything else out first and leave you in there for 5 minutes and lose all mass to get a vomit RNG tick in the indigestion RNG ticks)