A short answer detailing the possible reason behind swans having different colour feathers.
Why don’t all swans have the same colour feathers?
There are seven different species of swan, five of which are white, one is white with a black neck and one is black. The colour of animals can be influenced by a number of reasons, these include the need to hide from or deter predators and to attract the opposite sex. it is likely that for the species of white swans that are all found in the northern hemisphere that the white feathers are used to help with camouflage due to the often snow or icy landscapes that are common place in the northern hemisphere.
However it is less clear as to why black swans are this colour, while in the southern hemisphere it is clear that white would be a less desirable colour for camouflage with the limited levels of ice and snow. However, why black for southern hemisphere swans is less obvious, the Australian landscape does not lend itself to the idea of black feathers for camouflage as the habit is not black by nature.
It has been suggested that dark feathered swans are caused by the pigment melanin, this pigment makes feathers stronger and protects feathers from wear and tear, this would suggest and evolutionary benefit of black feathers but it would not explain why only southern hemisphere swans would require these. Melanin also protects skin against the damaging effects of UV radiation, which would be a greater threat for the darker swans found in Australia and New Zealand. This could be related to the fact that the legs and feet of swans are also black.