QmlDesigner.NodeInstances: Fix color reset value

There is the color property with the r, g, b, a sub properties.
Before this change we only saved the default of the sub properties.

Change-Id: I11d4b0ed83cf8469836099a4f65942d7e8c8d8a6
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>