Why Japan's ambassador could not warn Washington about Pearl Harbor

Would decoding Japan's Purple diplomatic traffic have allowed its ambassador in Washington to give advance warning of the December 7 attack?

No. The ambassador could read the worsening situation, but Japan kept him uninformed about the actual operation, so his messages would not disclose the attack plan.

The Japanese ambassador could see that relations were moving toward a crisis, but he was not told what the armed forces intended to do. Tokyo deliberately kept him outside the operational plan. Intercepted diplomatic messages could therefore convey tension without supplying advance warning of the attack itself. This separates strategic inference from access to operational intelligence.