It's hard to say. Im surprised the lava rock is disintigrating already. The color change is probably due to it absorbing organics in your aquarium water.
Most people have great luck with granite. Keep in mind, the more porous the rock, the more likely it will be to erode over time. Granite and other igneous rocks are significantly denser than lace rock. It really depends what's available locally to you, as shipping rocks is... well, heavy.
If you decide to keep what you have, an option would be to take the rocks out, give them a good scrubbing (Id stay away from even a diluted bleach, since you may never get it out of the rock), and maybe try covering the rocks with flame moss. It's probably not the look you were going for initially, but attractive in its own right.