This morning my good friend Andrew sent me this short piece published by the Ottawa Citizen by William Watson. The columnist is a McGill University economics professor who studied the image behind the Liberal team's riding photos.
All of the candidates wear some black shirt or blouse to match the red and black background. It's also important to note what Watson who stated the photos were most likely Photoshopped onto the candidates. If you go on their Liberal website they are wearing completely different clothing.
From my perspective, it seems as though the Liberals want to create a universal image so it appears they are more of a team, rather than individual candidates.
Black is professional, formal, and you look more at the face than the item of clothing they are wearing. They are wearing the clothes, the clothes aren't wearing them. To me, it seems they would rather make their personality shine and give emotions through these photos which focus on the face, rather than their entire ensemble.