I forgot to mention that in your second screenshot you have your Curves effect applied before the Audio Spectrum effect. So, even if it were to work, it needs to happen AFTER the Audio Spectrum effect.
You need the color of the resulting audio spectrum to be grey. One way to do that is by applying a Fill effect after the Audio Spectrum effect.
This tutorial tutorial may give you some better ideas since it looks like what you're trying to do.
Also, you could use the new (free) Color Vibrance plugin from Video Copilot without using the Fill Effect.