Showing that 3-colorable is NP-complete
No, you couldn't say that, because it's not true.

A helpful method is to prove all your claims.  Don't just guess -- try to find a proof.  If you're struggling to find a proof, the first thing to check is whether maybe the thing you're trying to prove is false, so look for a counterexample.  In this case you should easily be able to find a counterexample to your claim.  Alternatively, if you can't find a proof and can't find a counterexample, look for a different approach or a different way to solve the problem that uses only facts you do know how to prove correct.