Depends on what scale you play with. These mats will be primarily for 6 mm and 3 mm figures. The bases and the background mat thus need to be as light-colored as possible in order to reflect as much light as possible up into the figures. The main choices are thus very light green, yellow, or tan.
If you are playing 15mm or larger, dark terran might actually higlight your figures better.
We get slapped around, but we have a good time!