Mariner 10 did not take any color photos, according to an article by Andrew T. Young of San Diego State University, published in Sky and Telescope, May, 1985. In that article he provided swatches of color for most of the planets, along with the Moon, Io and Titan. The swatches were generated from reflectance spectra that were not subject to the limitations of many of the spacecraft.

I will go into it more in a later post, and I will try to scan and upload the swatches.

The swatch for Mercury was much browner than the neutral gray on the black-and-white image shown in the linked page. Let me add that those pages make no claims about the accuracy of color rendition.