What is Opaline?
Opaline is a sex-linked gene.
The wings of a normal blue series budgie have black bars on white wing feathers. Normal budgies of the green series have black bars on yellow wing feathers.
Opaline budgies do not have this this white coloration on their wings.
As shown in these pictures of sky blue birds-
The body color of an opaline budgie will take the place of white or yellow on the wings.
If you have a green Opaline bird, she will have green on her wings where the yellow would be on the wings of a normal green budgie (below).
Opaline's have two other identifying features.
The other identifying feature is a "V" of body color on the upper back. As can be seen in the sky blue Opaline's picture, the barring stops or is reduced above the wings, making a "V" shaped area of plain body color.
Normal sky blue female
Normal Opaline sky blue female*
There is usually a lack of barring on the head of the bird. This is not always true though, as seen in the sky blue Opaline above. I believe that a bird good enough for show should have little or no head barring (like the green Opaline above).