they have to be at least great or good but if there wonderful there will be a larger chance of producing a blue or green chocobo. their racing rank at the gold saucer also effect the chances of of special chocobo being born so a male and female wonderful chocobo both as rank A or S(it doesnt take long to get to A with a half decent chocobo.) there will be a near 100% chance of a green or blue chocobo if you use the right nut when you breed them(just incase you didnt know, you need a carob nut or it will just be yellow.)