Bring Boursin cheese to room temperature or microwave for 10 seconds to soften. In a small bowl, stir until soft and spreadable.
Line a Halloween mask with plastic wrap; make it long enough to fold over the front of the mask as well. With a rubber spatula, press the Boursin cheese into the mask to make an impression. Wrap the extra plastic wrap over the cheese and press gently with your hand to reinforce the impression. Take the wrapped cheese out of the mask, unwrap and flip the mask over onto a plate. Discard the plastic wrap.
To color the face, with a pastry brush, brush green pesto all over most of the face. Brush the rest of the mask with red pepper pesto.