My guess is it is a slightly blueish slate colour, but only the person who coloured the picture will know.
There are good whites which reflect at all wavelengths, and good blacks which absorb all wavelengths. There are good greys which absorb a similar fraction of all wavelengths, and there are a few anomalous greys which do something more complex. It would not be difficult to balance the camera RGB channels so good greys give pixel values where R=G=B.
The martian camera RGB spectral sensitivities probably do not match the human eye. The primaries may have been chosen to give the best colour separation (our ‘red’ channel and our ‘green’ channels overlap a lot), or the best information for a geologist. Giving us an accurate simulation of what a person might see is not a priority. So, camera RGB will not give us what we see, even after a 3x3 matrix correction: you need the original spectra, and that extra data was lost if the camera only recorded three channels.
The dunes are illuminated by sunlight, which has a fairly smooth spectrum even after passing through the thin martian atmosphere. They may be slightly on the blue side of grey and the enhancement has made it bluer. But there is no grey reference to compare it against, so I cannot be sure.
