Thank you all for your comments and encouragement. I really appreciate it.
Time for another update. I went ahead and got some of of the buff cement colorant and was able to blend a much nicer brown color. After adding a little extra brown shading to the roots I'm much happier with how they look (I think the originally color was actually more purple than it appeared in my photos).
I've also covered the walls of the emersed portion with Ecoweb, which is an artificial hydroponic planting medium used by orchid growers. It's essentially a really rough mesh composed interwoven plastic fibers (similar to an abrasive scrubbing pad). Since I'm going to have a constant flow of tank water circulating through the Ecoweb the entire wall effectively turns into a wet/dry filter which means this tank is going to have
ridiculous biofiltration!
Some people have had great results getting lush moss growth on this stuff so I'm hoping it will work out well for growing moss and epiphytes in my tank.
Cutting it up was a bit of a pain but I think it worked out pretty well in the end - here's how it looks:
I thought the Ecoweb was supposed to be black but it's actually brown (quite similar to the color of my roots). It means there's a little less contrast with the roots but it does look more natural. Hopefully it will all be covered in moss/plants anyway so it shouldn't matter. Once I've got the water circulation system running I'm also going to incorporate a little bit of clay/peat/moss mixture into some parts of the background to hide the Ecoweb seams and provide a little bit of organic planting medium.
I left the walls of the submerged portion blank so I just have a nice, simple black background underwater.
You may notice that I've also added in my drip lines/spray bars at the top of the tank. As I had planned earlier, I used two lengths of 3/4" PVC (one for the back and one for the side) with holes drilled every inch or so facing towards the background. This should hopefully provide a nice even flow of water down the Ecoweb. I painted the PVC with two coats of black tinted Drylok so it wouldn't stand out so much.