I think it depends. Avenue Q and Spelling Bee recouped right away and still have color Playbills. Sweeney also recouped quickly, but still switched to B&W.
Well the shows mentioned: Q, Wicked, and Spelling Bee are all doing extremely well in grosses. Just like Jersey Boys and Spamalot (last time I saw one of the ground)
Both times I have gone to see Wicked the playbill is in color. Because they are making tons of money.
I went to see the 27th preview of Sweeney Tood, and the playbill was in color. A few months after it opened, they started printing it in black and white.
It really depends on how much money the show is making.