Dark brown voice?

Makes sense to me. A dark brown voice would be deep, smooth and
thrillingly sexy. The words themselves have a swooping drop and rise
motion to them. 

IMHO.

Isn't there anyone else out there who associates words with forms,
textures, colors and movements?

Evidently Ray Davies of the Kinks does, since the phrase in question
comes unchanged from the original song "Lola".

I had always thought that it was "backroom voice"...
But then, I always thought that "Lazy Old Sun" was a much better song
anyhow

Terry