1. The latter is more likely. Your body has learned to run off "good" foods and no longer processes the bad ones. In a way, you lost your intolerance to those foods.
2. White would be least distressing, in most cases, but you should experiment to know for sure. Try not to go overboard, because you WILL experience side-effects either way.
3. Yeap. Severity/length varies from person to person and food to food. For me, a huge trip off the wagon will result in several days of feeling off, after an initial absolute crash (feeling flu-ish). A small one may only last a day, with gas and bloating.