If you aren't going to be painting your base boards, therefor leaving them brown, I would still paint the crown a creamy white. I say creamy white so that they aren't too stark white with the base boards.

Paint the crown the same white as the rest of your trim is throughout the house, which I assume is a white.

Here are two photos. One shows dark base boards and doors with white casings and crown, and the other one show dark base boards with dark casings and white trim. Hopefully these will help you decide which route to take.