Can the images used be changed manually regardless of gtkrc present? For example, by replacing the images in a folder?
It is possible to change the colour of the minimise and unmaximise buttons as they both follow the panel text gtk colour. The close button always remains orange/red.
It is also possible to get the panel to draw ANY buttons you want but to do this the Metacity theme and gtkrc being used must be called Ambiance and follow it's button naming schema.
Any metacity theme in the home/(username)/.themes may be used as long as the gtkrc being used is still called Ambiance. This means the panel will draw the buttons from the folder usr/share/themes/Ambiance/metacity-1 as long as whatever theme being used is called Ambiance (does not matter if the gtkrc resides in all users or personal theme folder)
The downsides to all of this are that:
a) Only one theme may be set up to properly draw the panel buttons at a time.
b) It must be called Ambiance which can be confusing and means that any decision to try a new theme that draws the panel buttons requires overwriting the Ambiance metacity in usr/share/themes.
It would be a lot better if Unity had the ability to probe any theme under usr/share/themes for an appropriate Metacity and only go back to the fall back buttons if it didn't find one.