Actually, it will depend on what activity the vehicle is performing. Has it been in the field, running cross-country, sitting in a motor pool, or is it a derelict wreck? Additionally, what environment is involved? Tracks have become a pet peeve of mine after 38 years in the Army/Army Reserve with about half on and around tracked AFVs. I apologize, ahead of time for the rather wordy response.
1 - With a few exceptions, tracks are made out of a manganese/steel alloy and as such they rust. However, when operated in their natural environment, the soil will polish the metal components. Here is the life a a track: It starts in the motorpool where is is a medium brown rust color (similar to the colors described above), then after a day of roads, dirt trails and cross-county operation the rust is all gone and a bright dark steel color is visible (recommend you look at an active bulldozer at a construction site, or photos on-line). At the end of the exercise or when pulled out of the line for a few days the vehicle gets washed (be it a wash rack or a local stream) as you cannot perform proper maintenance on a dirty vehicle. As soon as the vehicle is pulled from the wash site, the track with get a bright rust orange "glaze", the more humid the environment the faster the color change. By next day the track has a solid orange-brown patina that gets browner day-by-day, until it goes back out to the field and the cycle starts again.
2 - I tend to paint "active" vehicles. The type of track will determine if I use a primer. On old Tamiya-style rubber-band track I will use a water- base primer and acrylic paints. For metal tracks I will normally use a primer, type is dependent on type of overcoat. For injected plastic I can skip the primer. I start with a dark steel color. I prefer Vallejo Model-Color Gunmetal Grey. It provides a good dark metallic base. I will use a color like Vallejo Model-Color Steel to highlight high points like the track grousers that come in direct contact with the ground. This highlight can be added before and after any dirt. For a less stark contrast where it more rubbing than grinding you can use rubbed on graphite. For dry ground like dirt roads or desert terrain I use something like Polly Scale Earth which is thin and will settle into low areas. It may take several coats. For wetter soils I discovered "texture paint" from Citadel, which are great for packed dirt and mud. Several colors are available dependent on where in the world your vehicle is operating: "Armageddon Dust" (a dark sand) which I used on a Dutch YPR-765, "Stirland Mud" ( a dark brown)for Belgium/French mud on my WWI Mk IV heavy tank, and "Blackfire Earth" (orange-red) which may be good for Vietnam. These textures are a thick paste, thicker than paint but looser than putty. You can also add vegetation depending on your scene.
3 -As Chris mentioned above, more than the terrain acts against the tracks, Wear plates on the insides of road wheels against center guides (HVSS suspension on Shermans) or edges of road wheels against the guides on edges of tracks (early Sherman suspension), and wear on end connectors and edges of track from sprocket teeth. The center guides become really polished so some type of bright steel is what I use.
4 - Finally, don't forget any rubber parts which are common on American WW2 vehicles and most modern US and European vehicles in countries with a lot of paved roads. Rubber pads on outside of track reduce road damage and wear on metal track parts while rubber on inside surfaces reduce wear to rubber tires of road wheels/return rollers, but also make a quieter track. After painting the rubber parts with Polly Scale "Steam Power Black" (flat black is also good) and the paint has dried I hit the rubber parts with a sanding stick to rough up the surface. I may also rub in some sand colored pastels to represent ground-in dirt.
5 - Track Brown is an excellent color to represent old rust, or can be used as a base, as described above, with steel/gunmetal gray on high points to represent track run on hard-surfaced roads such as a parade vehicle.
Again I apologize for such a long answer.
