Wepwawet’s general coloring is considered to be black (which Terence DuQuesne, foremost Egyptian-jackal-God expert, did not hesitate to state plainly in his book Jackal Divinities: “[Who] was almost certainly coloured black”), the same as the jackal Anubis. In some cases, the coloring may vary, particularly in jackal-headed form and on certain funerary depictions where alternate colors appear throughout (blue, grey, or yellow as examples).
In these cases, it’s important to note that Wepwawet is not always depicted as a ‘different’ color; rather, the coloration of the jackals throughout the same depiction where black is not the primary motif will most often be consistent within the same canvas.
One example is the Djehutymose Coffin where Anubis, Wepwawet, and Duamutef jackal Gods all appear on the same coffin lid in a similar shade of faded greenish-blue [source]. In other words, if Wepwawet presents as having an alternative color or a complete lack of pigment, so too does Anubis or other present jackal Gods.
There is a prevalent misinterpretation of Wepwawet being depicted as a grey, white, or otherwise non-black jackal ‘in contrast’ with that on Anubis; however, the evidence of this is limited, and Wepwawet can and should be referenced as a Black jackal (Wheatley, Opening the Way), the same as any other. Keep in mind, the palette of ancient Egypt primarily revolved around only 6 colors to begin with: green, red, blue, yellow, white, and black. The concept of the color ‘grey’ may have been more closely associated, if not synonymous, with black to begin with, making the idea of a ‘grey’ jackal all the more unlikely.
In addition to the color’s existence being somewhat unsupported, we also have to consider the problem of pigment degradation — that is to say, the absence of pigment should not be automatically attributed to ‘grey’ or ‘white’ (Wheatley). In general, a common form of black pigment was made from soot (called lamp-black), and depending on the stabilizing agent used to make the final pigment for application, its longevity can be suspect; the pigment, falling away after many years, would leave behind a grey residue or mark. At sites where Wepwawet is present with a discolored head, the degeneration or poor adhesion over time of the black pigment must be considered, particularly where similar adhesion or degradation issues exist in other areas of the or closely-linked painted medium(s). This was compounded by the poor preservation practices of early explorers.
This ‘grey’ coloring idea has also led to the adoption of some additional misconstrued ideas about the Deity, namely that He was represented not as a jackal, but as a wolf, particularly in the days of very early explorers, scholars, and the infancy of Egyptology in general. This kind of citation appears in some common sources for modern-day Egyptology knowledge and it has presented itself a challenge to defeat in the public eye.
There are some examples where Wepwawet appears with a different color or absence of pigment vs. another present jackal God (such as Anubis) that I cannot and will not deny, though even in those instances, it’s unclear what the original color may have been. Often what’s left today is a bit of mottled greenish color on a stone background, but that isn’t to say the color was grey. The way the ancient materials degrade, it could have been a variety of things: blue, green, or yellow pigment, or even some kind of metal gilding. These case are however more rare than not, and we also must consider that the sheer volume of material for Anubis depicted in Kemetic content leads to a logical disbalance in what we find has survived intact after all these millennia between the various jackal Deities.
As a personal opinion, I don’t believe the idea of ‘faded pigment’ can actually account for all instances of Wepwawet being a different color than Anubis or other Jackals in these circumstances; in fact, I would think it somewhat a normal concept to differentiate two somewhat-identical beings in the same image using more than name or inscription alone.
There are some notable instances of this, such as the Cartonnage of Nespanetjerenpare (ca. 945–718 B.C.E.) where an obvious distinction of color is made between the two, standing next to one another. This cannot be attributed simply to faded color, considering the same color was used in rhythmic areas of the border motif as it was for Wepwawet’s hybrid jackal-human form, and considering the clear black pigment of Anubis and some surrounding inscriptions again remains completely intact. The pattern-nature of the matching colored border alone makes it an unlikely accident, far more likely to be an intentional distinction.
As a result, the coloration of Wepwawet’s jackal should not be fully presumed as 100% one way or the other. What is important to note in any case, is that we should not draw any premature conclusions about the sAb animal / canid / jackal itself from these color-points alone.