As far as I know, the only difference is the color. Sony Ericsson, as well as most companies release their handsets in different colours to suit different tastes.
Yeah it turns out it’s also a feature that you get with different Networks, e.g., if you were to buy it through 02 you would receive the blue one. Whereas if you purchased the Vodafone branded one you would get the green one.
in some countries certain model of phone colours are only brought in but certain networks. it happened in my country for k750i last time. theres no difference.