how to draw simple coloured objects

I'm trying to write a short GUI to allow the user to put together an animation script. The user should be able to drag animation "events" up and down a control.

My original approach was to try and use the Unity GUI system to construct the control out of individual box/button controls. What I want is to be able to draw a simple, partially opaque coloured box but I can't work out how to get the GUI library to do this, the default background colour seems set to black.