Mulch is actually a great way to keep the soil moist and weed free. Just don't pack it against the tomato stems as that is known to cause problems-leave a couple inches back.

I actually have also had a question about this, will using colored or treated mulch leach chemicals into the plants? I see in pictures people using mulch on top of the earthtainers, but was hesitant to use any mulch, especially in earth tainers, because whatever is leached out is just going to keep getting recycled

Never have I seen any damage to plants with the use of colored mulches. They've been out for quite some time and if there were a problem it would have gotten circulated by now.