The most abundant minerals in rocks in the solar system are white oxides, except for the iron(II) and manganese oxides, which are blackish.

So most rocks are gray, unless they have been exposed to oxygen, which converts the black iron oxide into red iron oxide, in which case the rocks turn from some gray shade to some brown shade.

Where there has been water, the water could have been dissociated by light, providing oxygen for the change of color. In the absence of water, there are few chances for anything else that could change the color of the rocks from gray.

BepiColombo has an X-ray telescope on board which will measure the X-ray colours of the planet from X-ray fluorescence. This will be interesting, as it will enable the composition of the surface to be measured down to 10 km scales [1].

[1] https://link.springer.com/article/10.1007/s11214-020-00750-2

It's even greyer than the Moon apparently. Unless this mission finds an active volcano, which I doubt it will, or some other colorful feature hidden from the previous missions, I think that's pretty much it, nothing but shades of grey.

https://photojournal.jpl.nasa.gov/targetFamily/Mercury

It has some subdued colours such as brown and light blue. Here's a previous NASA image:

https://apod.nasa.gov/apod/ap110331.html

Here's an image with exaggerated colours:

https://apod.nasa.gov/apod/ap110616.html