Why does the color on the deleted scenes alternate between black & white and color?

Sometimes the deleted scenes were never finished, so the color correction process may not have been done.

Could be the trend I've seen with other discs. The black & white sections are usually the scenes that would have come before and after the deleted scene as they appeared in the movie. The colored scene being the deleted scene itself. It's a way of showing you the deleted scene in the context of the movie and helps you to not have to play the "what's different about this scene" game.