Rach, we had a good debate a while back on what makes plants shift red. while the majority seem to do so as a reaction to high light, there are many plants that go red from a lack of nitrates and/or phosphates. I suggest you measure the nitrate and phosphate levels normally found in your tank. Let us know if either of them are at or below 10ppm, and then we can suggest plants that red up the most in a low macro-fert environment. for example many have noticed that Limnophila aromatica go red/purple more in a low nitrate environment, while Hemigraphis Traian red/purple up best in a low Phosphate tank. with both plants light still plays an additional roll in reddening them up. You should also consider many low light requirement Cryptocoryne that have reddish brown leaves such as wendtii and beckettii, however you may not consider those "pretty" enough. the truth is 2wpg is really "medium light" and many normally red plants such as Alternanthera reineckii and Ludwigia repens should maintain their red display in your tank.