"First off, I am going to point out that the late Terence DuQuesne, THE foremost expert on jackal deities, adamantly agreed with me that Wepwawet was not depicted as a grey or white jackal. Claims of such have very little basis in reality and for the most part seem to arise from some all too common oversights and misunderstandings.
The biggest of these misunderstandings seems to be the depictions of Wepwawet in the temple of Seti I in Abydos, where much of the relief retains colour except for Wepwawet’s head. But it is very important to stress that a colourless depiction does not equate to ‘white.’ It equates only to what it is. The overall lack of surviving black paint at that temple has been noted by scholars. From my reading, the fragility and poor adherence of the black seems to have been caused by a reaction between the pigment and the underlying plaster. This temple, along with others, were also subjected to archeological squeezes, conducted during a time when egyptologists didn’t really care that much about preservation. The poor survival rate of black pigment when it comes to temple reliefs isn’t just limited to Abydos either. And on those reliefs where the pigment on the face and/or body IS surviving? Yeah, it’s black.
The second major source of misunderstandings about Wepwawet’s colouration may come from the painted murals within tombs and decoration on funerary equipment. A) Tombs and sarcophagi do sometimes feature unusual colour schemes, so you might find green-skinned, yellow-skinned, or blue-skinned jackal deities. It tends to be consistent however, with no differentiation in colouration between Anubis, Duamutef, Wepwawet, etc, that I have noticed. B) Sometimes the pigment used as black in a particular tomb does now appear to be more grey, perhaps through degradation or just because it wasn’t painted on thickly enough or what-have-you. But that means that the grey is not really meant to be grey, it is meant to be black. There might be a grey Wepwawet, but other commonly black elements will also appear to be grey. This does not a grey Wepwawet make.
A friend of mine brought up a third point when she noticed there are more depictions of Anubis with surviving pigment than there are of Wepwawet. She felt this probably has an impact on people’s perception of what colour Wepwawet is, and I agree that it may very well do! It is, however, another case of not misinterpreting 'colourless’ for 'is coloured something other than black.’ He would have originally been painted black. An interesting thing about depictions of Wepwawet is they are quite commonly found in areas where there is a greater exposure to the elements (such as temples) — as opposed to Anubis, who is often is much more sheltered locations such as tombs or inner rooms. It is only logical that the pigment would survive better in areas of less exposure. And, as I’ve implied, black does also seem to be one of the first colours to go when it comes to temple reliefs.
I must also say that I have yet to find a single instance/location where Wepwawet and Anubis are actually painted different colours, let alone enough examples to declare it as an established pattern. (As I pointed out, unusual colour schemes can and do happen but they should be considered anomalies. Or a contextual variant if there are enough examples.) It is therefore deceptive at best to say 'Anubis is painted black while Wepwawet is painted grey.’ I appreciate that a lot people have gotten this idea from Wilkinson’s 'Complete Gods and Goddesses of Ancient Egypt’ and so I actually contacted him about it awhile back. He had this to say to me: “My comment on Wepwawet was based simply on my own experience - that a number of examples I have seen (inasmuch as it is possible to tell that W. is intended) seemed to be grey or not to be painted black when color was otherwise present. On the other hand, looking now at what I wrote, I think that the implication that this is "usually” the case is due to a poor choice of words on my part. I believe that what I meant was that while Anubis is usually shown as black, W. is sometimes depicted as grey (or without black). It is interesting if you have not found any examples of W. that differ so from Anubis - perhaps it is a rarer phenomenon than I realized.“ (Sadly, we were not able to get into more specifics at that time.)
I have addressed the lack of black and also the occasional appearance of 'greyness’ in those situations where it is serving as a substitute for black or else is suffering from degradation or transparency. It seems relevant to add that green can also degrade to a quite greyish look and is a not uncommon skin-tone in funerary contexts due to its colour symbolism. The thing about grey as a colour is it does not have its own individual symbolism in AE art. It classifies as black. So it doesn’t even make any sense, when colour is always used in such a highly symbolic way, for the Egyptians to have distinguished Wepwawet as being grey while Anubis is black. It also comes into conflict with the fact that since the Old Kingdom, the vast majority of jackals, be they gods, spirits, or even just hieroglyphs, are painted in black."