Cinnamon cockatiels (shown on the right) have grey-brown feathers instead of the dark grey feathers of a normal bird. Their eyes are dark, and their beaks and feet are greyish. Cinnamon is a sex-linked mutation.
Fallow cockatiels (on the left) are similar to cinnamons, but their eyes are red, their beaks and feet are pink, and their brown feathers are paler and yellower than cinnamons’. Fallow is a recessive mutation.