I have Revit 2012 ans whilst I can see all the colours in "realistic" mode of the materials that I have used, when it comes to generating a rendered presentation image Building materials comes out grey except the grass and trees. Can anyone tell me why this is happening and what the setting is to resolve it?

You almost have to set the rendering colors for each material for things to turn out correctly.  It is a trial and error, at least for me it is.  I don't think there is any magical setting that will do what your asking.  Make sure every object in your model has a material applied to it.  If nothing else works use the paint to try to get the colors you want.   Good Luck

Thanks for the feedback, i'll try that.