You don’t have to go in search of a magnifying glass to discern the active ingredients in the new musical “War Paint,” at the Nederlander Theatre, a dual biography of the dueling cosmetics divas Helena Rubinstein and Elizabeth Arden. The magic elixirs are quite plainly the two veteran Broadway stars above the title, Patti LuPone and Christine Ebersole, giving performances of such resplendent force, wit and vivacity that the evening gleams like a freshly applied coat of nail polish catching the light.