few points I wish to add. This applies to pc version if relevant.
First I think the random number generator in the game clearly is glitchy, random events can be umm non random. Hence getting same chocobo repeatedly.
You can get 2 different coloured chocobos green and blue from same parents, I have done it in my current game and was very lucky to get what I wanted first time, ie. female green followed by male blue.
Between breeding you need to do random battles to pass the time, racing does not pass the time, but of course winning races is needed to garunatuee the right chocobos.
Breed great with great, no need to do great with good as sade in various guides.
Feed at least 10 sylkis so speed is at max potential, this gives you a better stat new born chocobo.
Now the rank to race to?
Just about every guide I read says race it to S rank and that gives you best chance of getting a special chocobo however my experience is different. To me A class is the pinnacle, get the chocobos to A class and you will always get a special chocobo out of the carob nut every single time, if I race to S class, I only sometimes get a special chocobo so it seems the game mechanics punish you for reaching S class and breeding. (this goes against all the guides I have read including the choco mechanics).
Race the 2 coloured green and blue to A class and of course give them at least 10 sylkis greens before breeding.
Give them a carob nut and you should be garuantueed a black.
Race the black to A with 10 sylkis greens or more and breed with a wonderful using a zeio nut that is also A with at least 10 sylkis greens and you get a gold garuantueed. again if you race to S class its not garuantueed and can get normal chocobos.
Now my problems came when I wanted more than 1 of black and gold chocobos so I can breed them with each others to get the fabled 176 speed chocobo, the game mechanics seem to change once you have your first gold chocobo, I start getting normal chocobos even on A class chocobos.
If you find it keeps spitting out the same sex chocobo and/or same colour you need to fight some battles to change the random generated number, when I kept fighting battles outside of the ranch it made no difference but after I did a fight in the crashed gelnika on harder monsters the generated chocobo was changed and I got what I wanted, so I guess who you fight makes a difference or perhaps moving to different certian areas generates a new number.
