The Yellow Beak mutation changes red lipochrome into yellow lipochrome. Both the beak and the legs turn to yellow.
This is an autosomal recessive mutation.
The Yellow beaked mutation is recessive (autosomal). Notice the difference between the two beaks on the picture above. Both birds are males.
When buying Yellow Beaks, please note that zebra finches kept in poor light will sometimes lose some of the colour in their beaks which makes them resemble the true Yellow Beaked mutation without being so.