here are the things u need to know.
1.) First, you need two chocobos, 1 great and 1 good. The great can be found neer mideel when u fight two spirals w/ the chocobo. you can also find the good next to the gold saucer with the two spencers alongside w/ the chocobo. Next, mate the two chocobos (remember they must be the opposite sex) with a carob nut. carob nuts can be stolen from that red dinosaur guy near bone village. I believe if the great chocobo is the male and the female is the good, u get a river chocobo (blue). vice versa to make a mountain chocobo (green) (however, i have done it before where the i used the same two chocobos to make both the river and the mountain.)
2.) to acquire a black chocobo, u must mate the river and mountain chocobo w/ a carob nut. make sure both chocobos are at high rank levels (at least A) for the chocobo racing. To make a fast chocobo, feed it 15 syllikis? greens that you can buy from the chocobo sage near icicle inn.
3.) to acquire the gold chocobo, u must mate the black chocobo w/ a wonderful chocobo, which can be found near the icicle inn fighting along side with those two bunny enemies. Increase their ranks and mate them w/ a zeio nut (which can be found in the north-eastern part of the map on an island called goblin island where you can steal them from goblins). If you follow all of these steps, u will receive a gold chocobo.
Mountain---------you can get the Mime materia near wutai
River---------------you can get the quatra materia near mideel
Black (mount+river)---you can get the HP-MP switch materia near the gold saucer
Gold-----------------Knights of the round summon in the north-eastern corner of the map