This can be done very quickly using the Attribute Manager. I encourage setting up default materials in your template file, that are assigned to each building element. Eg. Default Roofing, Default Flooring, Default Ceiling, Default Object, etc. These default materials can be simply copies of 'Whitewash'. This way when you first render you model, everything is just white, similar to a styrofoam model 'from the old days'
. The client doesn't get distracted by the crazy colors you've picked and you can very quickly swap out materials globally, using the methods described in my tip. I will post this as a separate thread in 'Working in ArchiCAD', here in the next few minutes...
Cheers,
Link.