Hi,
You edit the mass, select the form entirely, and set the material.
As simple as that.

You can also Paint the faces with materials... Split Face works too for some details (for instance "windows" in a brick material)

To expand on jbenoit's suggestion - not only can you set each mass form a material directly, but you can also assigned them material-parameters so you can control them through the type/instance rather than in the mass editor, so that way then you can control more items at once.

I'm also fond of using sub-categories (for all things, not just masses) - which give you a greater degree of control in views, so you can maintain global-appearances (through the method above) but also use overrides that dig a little deeper...

For example, I've been developing a mass family that has the standard mass form (which defines the typical mass floor areas and reports the OOTB area/volume/perimeter/etc) and within that I've nested a wrapping "envelope" mass family, and an internal "space" and "service" mass family - all set to their own sub-category. Once in a project, I can then adjust what I see, (and how) in a number of different configurations.

Many thanks All