YELLOW FACE MUTATION
Yellow face mutation is a combination of white and yellow based mutations. There are few varieties of this mutation and it seems quite tricky to tell the difference at the beginning. In general - it's a budgie that is originally white/blue/grey with some yellow pigment added. And then it depends where in the body the yellow colour is present; it could be just a face, or the whole body. We also look at the shade of the yellow to determine certain variety.
Yellow Face Type 1, Single factor
It’s white based mutation (white, blue, grey series) where body is not affected by Yellow colour; only a head is. And the yellow is light/pale yellow. For example: typical sky blue mutation, where head is white and body is sky blue, now has light yellow head, but body is still sky blue. And of course, it can still be Normal variety, Opaline, Spangle, Pied, Greywing... the features of a certain mutation remain the same. The only difference is pale yellow head. So if your budgie is white/blue/grey but has pale yellow head - it’s Yellow face Type 1, Single Factor mutation.
Yellow Face Type 2, Single factor
This time not only a head/face is yellow, but the whole body gets affected by yellow pigment. So a bird that would normally have white face and blue body, now has yellow face and turquoise (greenish blue) body. Again, imagine adding a bit of yellow paint to a white paint, or blue paint. Type 2 is also a bit darker yellow than Type 1. And once again, Yellow face mutation can be combined with basic mutations: Normal, Opaline, Pied, Spangle..
Golden Face, Single factor
White based bird (white, blue, grey) has added deep yellow pigment (darker than previously mentioned Type 1 and Type 2 Yellow Faces). The yellow spreads to the whole body again (like in Type 2), so instead of white body, we see light yellow and instead of blue body, we see turquoise (bluish green, cold green). Very often the body looks more green on a chest and bluish on the bottom half of the body. This mutation is quite often confused with Dark Green or Light Green mutation, but in fact, it’s a blue bird with a yellow pigment poured over a blue body.
About the picture below. At first, especially if you don’t know much about mutations, you think all green budgies are ... just green budgies. It turns out Green mutation looks quite different to others. Golden Face, single factor is actually a blue bird with yellow pigment spread on their face and the whole body, resulting in blue/green, aka sea green. Natural green budgie has no hints of blue colour and its green colour is warmer, whilst golden face or yellow face mutations are cold green
The picture below shows the difference between Yellow face Type 2 and Golden Face, both single factor. As you can see - the bird on the left has medium yellow face and his blue body has been 'touched' by the yellow pigment too. Originally it was white and blue pied. With a yellow pigment added - the white looks like light yellow and the blue looks like turquoise. The bird on the right is also a blue bird that got treated with yellow pigment. His face is more yellow that the other bird's hence his name - Golden face. His body originally - blue, now looks turquoise, thanks to yellow pigment. You can see the bird is still blue in some places.
Below - the picture of three varieties of Yellow Face single factor mutation.
Yellow Face Type 2, Double factor
Face is medium yellow, body colour is not affected by yellow colour (or very slightly), so remains blue, white or grey. Very similar to Yellow Face Type 1 Single factor, but this time the yellow is a bit darker (not pale yellow like Type 1 Single factor). If you pay attention to details and have been reading carefully, you may have noticed I missed Yellow Face Type 1, Double factor and went straight to Type 2. That's because Yellow Face Type 1, Double factor looks like any other white based budgie (so the body is white or blue or grey) and there are no hints of the yellow pigment; neither in the face, nor in the body. The only way to know you have Yellow Face Type 1, Double factor budgie is by seeing its offspring.
Golden Face, Double factor
Just like the type before - Golden face double factor has unaffected (or slightly affected) body colour (white, blue or grey - all white based colours). His face is rich yellow - solid, intensive colour. The yellow pigment doesn't spread to the body. So the variety is pretty much like the previous one but the yellow on the face is darker.
Below - take a look at their faces colours. One is lighter than the other. The dark yellow makes a bird a Golden Face variety.