Whats big, green, and costs millions of dollars to put in a theater? The answer, of course, is Shrek, the Broadway-bound show being road-tested here in Seattle. DreamWorks initial foray into stage musicals takes William Steigs rude, ugly, but ultimately lovable ogre and his donkey sidekick and adds a whole bunch of songs by composer Jeanine Tesori (best known for her Tony-winning work on Thoroughly Modern Millie). Reworking the book and providing lyrics is playwright David Lindsay-Abaire, whose reputation was made with such dark comedies as Fuddy Meers and Wonder of the Worlda natural match for the whimsically ironic story. Advance word has it that this very corporate musical (through Sept. 21) is more than your typical Disney-style kiddie-fest. At a spotlight preview a couple of months back, Tesori and Lindsay-Abaire performed several songs, and their dark wit knocked the audience off their feet. (My favorite? A song by Shreks parents promises their son A big bright beautiful world . . . but not for you.) To protect their considerable investment (and reap more green), the producers have loaded the cast with some serious star power, including Broadway vets Christopher Sieber, Brian dArcy James, and Sutton Foster. (See my column here for an interview with the latter.) JOHN LONGENBAUGH Fifth Avenue Theatre, 1308 Fifth Ave, 625-1900, www.5thavenue.org. $25$90.7:30 p.m. Tues.-Wed., 8 p.m. Thurs.-Fri., 2 & 8 p.m. Sat., 1:30 & 7 p.m. Sun. Ends Sept. 21.