What does hamlet mean when he says to Ophelia, "God have given you one face and you make yourselves another" (III.I, 143-145)?

I'd say you have a pretty good grasp of this meaning..... In these lines, Hamlet is speaking to the use of cosmetics..... the ploys women use to snare men..... false actions..... what we would call phony behavior. Thus, Hamlet does infer that Ophelia is not as pure as she'd like to appear.