Real-life observations: First, the color of the pad will depend on age and environment. After some use in the field, the pads will fade to dark (sometimes not so dark!) gray, with a hint of the prevailing dry dirt color in the area. Once a rubber pad or track link has gotten dirty, it will rarely return to it's original black color without scrubbing and detergent. Since plain water is the most common cleaning agent, this doesn't happen much. I'd go with a dark grey/black...except for the contact surfaces.
The coloring of the contact surfaces will depend on the surface the tank is depicted on. Even a short distance on a somewhat hard surface (hard dry road, asphalt pavement, or concrete) will scrub the contact surfaces to an almost clean (nearly) black condition. So the pads may be two toned, with a dark face and lighter sides (non-contact surfaces). This same effect can occur on rubber roadwheels, after a longer run on a clean (paved) surface.
Also, modern replaceable pad tracks have a rubber insert on the inside surface of the track, on either side of the center guide. This is where the roadwheels run on the track. The rubber insert quiets the sounds of the track and running gear, and softens the vibration and bumps from the road. These rubber inserts are typically pretty clean, and still mostly black or very dark grey. The effect is of a dark grey/black 'clean' strip the runs down along sides of the center guides, even in very muddy or dusty conditions.