I just returned from a hunt in South Africa. I was looking for a common springbuck, but it wasn't a high priority. After bagging one, I sort of got into the game of trying for one of each color - copper, black and white, in addition to the regular. Collectively, they constitute a "springbuck slam", which I don't think has any official recognition, but is fun to go after. I certainly had fun getting all four (plus a Kalahari springbuck, which is the same color as a regular springbuck but seems to be a bit bigger, both in body and horns)!
As Andriesdeclerck said, these are just a color mutation and not a separate species or sub-species. Having said that, the copper, black and white (especially the white) seem to be a recessive trait (they aren't very common in a mixed herd), so that if you allow these to breed with the regular color, you will get only the odd color variation. I believe you will get more of the color variations if you segregate the colors and only allow them to breed with similarly colored animals. Having said that, I think most game ranchers would tell you it's way too much trouble to do that, so I expect the color variations will remain the exception rather than the rule.