figure background color when printing

[Ben Abbott]

When the figure background color was added for the gnuplot backend, David 
checked to ensure the color property value was numeric before setting it.

        http://hg.savannah.gnu.org/hgweb/octave/rev/1e38d9ed1e28

This is important because Matlab sets the color to "none" when printing. 
Unfortunately, Octave only supports numeric values or the color.

This change is beyond my understanding. Can someone take a look?

Ben