Sammy the last option, is to do a bit of Ruby scripting, and create a submenu or a toolbar that sets the selected object(s) material.
I posted an example:
[code] Set Layer Colors menu command example
It actually changes the layer colors according to their names. But it could be tweaked to change only one specific named layer color.
Or it could be tweaked to change all the selection set’s object’s colors.
Sketchup.active_model.selection.each {|o| o.material='Red' }
etc.