(This is an incomplete answer that needs improvement and investigation, but should provide someone who knows C++ with enough detail to find the real answer)
By default the Launcher uses a light gray semi-transparent color. When you hit invoke the dash, the entire thing takes on a blurred look based on the averages of your wallpaper colors. I asked Marco Trevisan where this can be modified, but it's in a bunch of places.
However, for the launcher when it's just sitting there you need to look in Launcher.cpp:
First grab the code:
bzr branch lp:unity
Then look in
unity/plugins/unityshell/src/Launcher.cpp for the
_background_color variable and adjust accordingly.