"The colors in the feathers of a bird are determined two factors: pigment and structure. For example, the green in some parrots is a result of yellow pigments over blue-reflecting feathers. In the case of a white peacock, its unusual lack-of-color is due to a missing pigment. This missing pigment is dark and absorbs incident light, making diffracted and interference light visible (i.e. common peacocks). The effect is similar to that of oil on water."
As quoted on Treehugger.