Am pretty sure that GREEN is a GOOD BUY and RED a good SELL. I make all my money buying and selling Fish, Grain, Clay mostly around 4 cities in a straight line, just going back and forth. until the sell price turns red, which is generally twice what I paid or more. For the above, I would routinely buy and sell for MORE than twice what I paid for it. The key was to have a crap load of pack animals to carry 8k or more product and my sales at any given city with a great price were routinely 5-15k gold. Then I'd buy things at a good price and continue the trek.
If prices went south for a while, I'd just rove around lancing looters for xp until the prices returned to normal.