[omk3]

Spoiler:

White.

If you pick two black -> black in pot -1, white the same
If you pick one black one white -> black in pot -1, white the same
If you pick two white -> white -2, black in pot +1

Whatever you do, white beans either stay the same or are reduced by 2. Since their number is odd, there will always be 1 left.

(@draghetto: edit to add spoiler tags, please)