Great question! When creating a shared room for a brother and sister, I would start with a neutral wall color. Then have each child indicate their favorite colors, and choose the two that are most complementary as the main accents. Use the sister's color choice in her bedding, and throw pillows, and the same for the boy's color choice for his bed. To keep the room from looking too divided in half by color, I'd use another neutral color (black, white, metallic) for the side lamps, organization bins and accessories. I'd then tie all the colors together with a coordinating rug and window treatment. Finally, to give each child a feeling of individuality and room-ownership, I'd hang large memo boards on both sides of the room for them to decorate as they please.