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This combination can be very attractive if the Isabel mutation is in the dark form. In some countries, and on the ZFD, the Grey Isabel is the dark form, while the Fawn Isabel is the light form.
Two Black Face Isabel males.
If Black Faced is combined with Isabel in the light form, all the black markings will be diluted so much that you cannot see that the bird is different from an Isabel without the Black Face mutation. See for instance Black Face Isabel Fawn.
However, if you combine Black Face Isabel with Orange Breasted, it will no longer matter how dark the Isabel factor is: the face and the belly will be orange. See for instance Black Face Orange Breasted Isabel Fawn.
The Black Face mutation causes the belly and the area between tear mark and beak to be filled with black pigment. Ideally, the belly should be as black as on the picture, but in real life, it is often less black.
The Isabel mutation causes black, grey and fawn pigments to be diluted, while the orange pigments remain intact. The degree of dilution can vary.