Firstly try switching the sexes. Any combination of great and good chocobos with a carob nut will give you a green or blue. It is random, so you can save and keep resetting if you want, but I always had the most success by changing the two sexes around. Also you should always have both on S class. So try that first. Also get the great chocobo from another spot. Great Chocobos can be found either on the tracks near Mideel or on the tracks in the Northwest corner of the Western Continent. I will pretty much guarentee this to work for you but remember always save before breeding. If you dont know how to get black and gold come back and Ill let ya know

EDIT*

I just remembered, if you want a gold chocobo the easy way beat Ruby Weapon, and the old man in Kalm will give you one for the desert rose