Yellowface budgies are in between yellow-based budgies and white-based budgies. There are different degrees of the level of yellow pigment, less than the yellow-based variety. These different levels of yellow pigment are caused by several different genes. Visually, there are two types of yellow face: Type I and Type II. In type I yellowface budgies, the mask feathers are all yellow. The yellow may also show up in the peripheral tail feathers. The yellow is confined to these areas only and the budgie is normally colored in the body feathers. Type II yellowface budgies have yellow in the mask feathers and tail, just like the type I. However, after the first molt at 3 months of age, the yellow diffuses into the body color and creates a new color, depending on the original color. In the case of the sky blue variety, as seen below, the type II yellowface creates a seafoam green color, but in the type I yellowface the body color remains sky blue.

Basic Genetics:
Complicated!

For information on yellowface genetics, see Gene Function in Yellowface Budgerigars by Peter Bergman

The yellowface type I variety has a bright yellow face but the yellow does not affect the body color or appear in the wing feathers.	The yellowface type II variety has a bright yellow face. The yellow mixes with the body color and diffuses into the wing feathers as well.