The possibilities are endless, but it seems that the new colours are only cases as the keys on all are black. Surely Nokia will attempt to appeal to a wider audience by adding more colours as they go along, but cases will be an easy way to add even more colours. White, Magenta and Cyan are primary pigment colours, Yellow too. Rumours have suggested that a yellow version is in the making.
The Nokia N8 came in seven colours (Silver, Black, Green, Orange, Blue, Pink and Bronze), two of those colours only launched several months after the initial release, so Nokia doing the same this time is plausible. Only time will certainly tell.