What leads do your lights have to solder to? I'm assuming you have a set that could be controlled by a DMX controller. The controller can maintain the color and brightness level. You'd just wire the 120V power to the DMX controller to the same light load of the overhead light that you already have or could be wired to a separate wall switch.
Technically you could also wire it so that the 2nd wall switch would be able to toggle if the accent lights were enabled with the overhead or not when it automatically comes on.
For the high voltage side, any wire should match your breaker rating, so 14 gauge for 15A or 12 gauge for 20A circuits. The low voltage side shouldn't matter much, I've used anything from cat5 24 gauge up to 18 gauge depending on distance of the run.