There are various ways a bird can be white.
The most frequent is, probably, recessive white which, when the bird has two recessive white genes, stops all colour from the feathers. A bird can carry one recessive white gene without showing any sign of having that gene.
Dominant white is another frequent gene causing white. Dominant white, primarily stops black pigment. Most all white birds which are dominant white, would have been black had they not had at least one dominant white gene. An otherwise black bird with only one dominant white gene tends, most often, to be white with black flecks.