This is the coloration of wild cockatiels, and it is dominant
to all color mutations (except dominant silver). All mutations
are variations of normal grey plumage, and are caused by dilution,
intensification, or absence of the colors found in greys.
The bird pictured is a male, which is evident from his clear
yellow face and bright red cheek patch. Female greys have grey
faces and more indistinct cheek patches.