Assigning Materials to the solids in model developed

Sir,

I was trying to assign material to the solids in the model. I am facing difficulties in assigning materials as the model kept on changing different colours and I am not sure whether my selection is correct or not.

And, I have created a pipe layout using various extrude and revolve commands, should I select each operation as a seperate solid for assigning material or can I select everything at once? I am facing problem as each extrude or revolve operation's output is acting as a different solid while assigning.