I read that if you want to experience rival heart events, that the girls cant have a higher heart level with you than the rival male. I haven't seen Ann and Cliff's black heart event yet and now Ann is at a purple heart with me! Is it possible to get her back down to a black heart?

Not true, I have all the girls on Red and I can still trigger rival events. You just need to become better friends with the rivals (in this case, Cliff).

It is possible for for the heart to regress (go back to a lower color). You can do it by littering a lot (although that will make everyone less happy with you) or you could try giving items that they really don't like.

it is true that when you answered the wrong answer in an heart event his/her hart level it happened to me in Ann's blue heart event and I answered wrong and her heart level goes back to purple