GREEN
Green is the original "wild" color of the Budgerigar.
There are three shades of green recognized:

Light Green
Dark Green
Olive Green

GREY GREENS
The grey gene combines with the green gene to produce the grey-green budgerigar.

The Grey Greens

GREYS
In the blue series, however, the grey gene completely dominates over blue genes and the bird is grey.

The Greys

VIOLET
The Violet gene is dominant over the non-violet (or normal) gene. Violet genes darken the bird by half a shade, as opposed to dark factors, which darken the bird a whole shade.
Violets

Recessive

BLUE
Blue budgies do occasionally occur in the wild. It is an allele of the green gene and is recessive to it. To appear blue, the bird must have two copies of the gene (gg).
There are three shades of blue recognized:

Sky Blue
Cobalt
Mauve