Found this reply to a similar question on one of the other forums, though this may be some help it was posted by Hop

Hi Jikki, I have an answer to the first part of your question, I try to race in the blue as much as possible but when your chasing titles and need certain races then I do race in the "good green" A LOT. The key to this for me is having a good horse that doesn't need to be whipped AT ALL. If you don't whip your horse then the rest the horse needs between races is shorter. For instance sometimes if you race a horse in the "green" and whip him til finish then the horse will probably be "gray" the next week. If you don't whip the horse then you will probably have a "green" horse the next week. Similarly it's not uncommon to have two consecutive "blue" weeks in a row when you don't whip at all (if the horse has decent health).

As far as your last 2 questions I have no idea as I have never let my horse sit long enough to ever get the "yellow/gold" or "red" condition.