When I'm  working with pastell I use different colored papers as painting ground.
Through the portrait will receive a colored background that changes the mood of the portrait.

Gray looks rather cool and calm, the portrait lives on in a warm vibrant orange / beige. However, the color of the paper may also have supportive effect. A white object whether dog, horse or cat comes on a colored cardboard to better advantage than drawn on a white sheet in pencil.

I am happy to advise you in the choice of background color.