How does a great-sounding story — the epic competition during the ’30s through the ’60s between cosmetic pioneers Helena Rubinstein and Elizabeth Arden — and two renowned musical actresses — Patti Lupone and Christine Ebersole — end up such an opportunity lost? We found out last night at the Nederlander Theater. Don’t repeat our mistake. “War Paint” was flat from beginning to end, the music uninspired and trite, and the story — frankly — uninteresting. The plot and pace suffered from taking essentially a “split screen” approach to the competitive moves these two titans made against each other, since they never actually met in person their entire careers. So the audience was treated to contertemps scene after scene. The actresses seemed to be trying too hard throughout, and the songs injected gratuitously instead of at the height of dramatic tension…of which there was little. It’s always nice to hear accomplished voices, but in this production, even they were disappointing.