Violet factor is a color-adding factor. However, it is not as strong as the grey factor. If a budgie has a violet factor, you may or may not know it. True violet only shows up on cobalt budgies (white-based budgies with one dark factor) or, if double factor, on sky blue budgies(white-based budgies with no dark factor). It is very hard to tell if yellow-based budgies carry a violet factor. The violet usually darkens the green of the body feathers similarly to a dark factor. Sometimes, if you look closely, a violet tinge will be visible on the body feathers near the feet and vent of a green budgie with violet factor. Sky blue budgies with one violet factor will have a violet tinge, especially in the body feathers near the feet, and sometimes look darker than a normal sky blue. It is very difficult to detect violet factor in mauve budgies.

Basic Genetics:
Violet factor - semi-dominant
Normal - recessive
Details on violet factor genetics...

Violet factor in a cobalt budgie or double-factor violet in a sky-blue budgie results in the true violet variety. The two budgies on the right are opaline violets.