There are 3 common yellowface mutations, but of these 3 there are 6 different appearances depending on if the budgie is a single factor or a double factor yellowface. To know what yellowface mutation you have, you must understand there are more than 3 ways they can appear visually.

This might sound complicated but it really is not if you look at 2 things: What color is the yellow? And how much of the yellow spreads onto the whole body?

The 3 common yellowface mutations generally have different shades and depths of yellow, with yellowface I being the lightest, Yellowface II being a medium shade, and goldenface generally being the deepest and most rich yellow.

Lemon Yellow

Buttercup Yellow

Golden Yellow

Single factor and double factor forms of each yellowface mutation effect the body differently, spreading more or less depending on how many yellowface genes the budgie has. Below are side by side examples of how each mutation looks in single and double factor forms.

Very little yellow will spread on the body of a sf yellowface I, leaving it mostly blue.

A large amount of yellow will spread on a sf yellowface II, making a sea foam green color instead of blue on the body.

A large amount of yellow will spread on a sf goldenface, making a sea foam green color instead of blue on the body.

No yellow will spread on a df yellowface I as the yellow is not visible in double factor form.

A little yellow will spread on the body on a df yellowface II, leaving it mostly blue.

A little yellow will spread of the body on a df goldenface, leaving it mostly blue.

Areas of the body that have no pigment (white) will usually have a light, medium or deep yellow wash depending on the specific yellowface mutation. Areas for example would be pied markings, the underside of the tail, df spangles or DEC's etc. 

Having a light yellow color in these areas does not mean that, for example, a sf yellowface I is actually a sf yellowface II when the rest of the body is not effected, so make sure to look at the yellowface color and how the body is effected as a whole. Below are some examples of this on white areas of the body.

DF Yellowface II Sky Violet Sf Spangle

SF Yellowface I Double Factor Spangle

SF Yellowface I Sky Dominant Pied