Thank you for all the advice everyone. So far, the Rage of Aquarius tileset is working out well, and I think I see a common theme to present throughout the level.

I tried PT32's suggestion of using a transparent black wall to allow for more variety, and I discovered that a careful application of walls behind it can effectively create three different dark tones of backround walls in progressing order. That should work out quite well, but I am still uncertain exactly what to do with the backround, as I originally envisioned the level as being underground or in the midst of some very extensive ruins.

If I go with the former, creating the impression of an underground lake while making the farthest layer of the backround believable will most likely be difficult. Simply painting it dark would not make it too appealing, I think. Any suggestions?