Ptah, the god of craftsmen has a green face. You can see him second from the right on this tomb fragment (more about that through clicking here).
But why the green face? In the literature it is often stated that it is because the green is a reference to the newness of the vegetation growing alongside the Nile after the annual inundation. Indeed he often has the epithet nfr Hr (Lüscher, 246) which fits. Nfr, is often translated as ‘beautiful’ but it has connotations of newness and youth, so a green face could indeed be described as ‘nfr’. The plant association seems to be at least partly correct as Tatenen who is associated with vegetation is also shown green, but then again, he is linked with Ptah.
He is also green as he is in a transformative stage, of becoming a divine form, a mummy. Mummy's were not just dead bodies but something more like statues of gods, indeed, like statues of gods they could be revived through the Opening of the Mouth ritual.
So, Ptah, the mummy is undergoing a transformation? So it would seem.
The whole idea of mummies being trasformed into gods is also made explicit by Chapter 151a of the Book of the Dead. This appears on several mummy masks, including, one of ours, W920. This Chapter describes the face of the deceased in terms of gods and indeed the whole process of mummification was concerned with making the deceased godlike. And so in Chapter 151a, the deceased is described as nfr Hr, and the Chapter is often illustrated on tombs, etc. by Anubis carrying out a mummification. Anyone in a stage of transformation is obviously going through a new stage and thus has to be nfr. Osiris, also sometimes shown as a mummy is also called nfr Hr.
Lüscher, B. 1998. Untersuchungen zu Totenbuch Spruch 151 Wiesbaden: Harrossowitz Verlag.