The Yellowface II budgerigar mutation is one of approximately 30 mutations affecting the colour of budgerigars. In combination with the Blue, Opaline and Clearwing mutations, the single factor Yellowface II mutation produces the variety called Rainbow.
2 Historical Notes
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The single factor (SF) Yellowface II Skyblue variety is like a normal Light Green but has a very bright body colour midway between blue and green — a shade often called sea-green or turquoise. The body feathers of the SF Yellowface II Cobalt are bottle-green and in the SF Yellowface II Mauve they are a mixture of mauve and olive.
The double factor (DF) Yellowface II Skyblue variety is very similar to the Yellowface I Skyblue, but the yellow pigmentation is brighter, and tends to leak into the body feathers to a greater extent.
Although not recognised as such at the time, it is possible that the first Yellowface II birds to be reported in the UK were bred by Jack Long of Gorleston-on-Sea in 1935. A contemporary report  of his breeding says, “Mrs Lait mated a dark green cock to a greywing mauve hen, and in their third nest was a pale greywing mauve hen with a distinct (light lemon yellow) mask and bib, with the under tail feathers yellow and with yellow on the wings in the places where the normal blue bird is white. This hen … was mated with a cobalt/white cock and they have produced five youngsters, all having yellow masks like their mother. Mr Long’s birds were bred from a dark green of a somewhat olive shade mated to a rather unusually coloured hen, which appears to be a green but has a turquoise suffusion on the breast, etc. The first nest produced 3 cobalt birds with yellow masks, etc, like Mrs Lait’s birds described above, and one green-blue bird like the mother. The second nest produced exactly the same result.”
The description of the birds suggests that Mr Long’s birds were a DF Yellowface II Cobalt cock and a SF Yellowface II Cobalt hen, but the breeding of Cobalts with yellow masks places this in doubt.
The genetics of the several Yellowface mutations and their relation to the Blue mutation are not yet fully and definitively understood.  
Much confusion and misunderstanding have arisen because the popular names given to these mutations are misleading. These mutations do not generate a yellow face, as the names might suggest. Rather th