I would start by focusing on the details of the Arc de Triomphe and the Colosseum. I would use darker shades to draw attention to the pillars and the relief sculpture of the battle. The shapes of the arches can be drawn out with a light pencil sketch and highlighted with bright colors to make it stand out against the center of the page. I would then use shades of browns and creams to highlight the sand floor of the Colosseum and then use a bright warm color to represent the sand. Finally, I would use blues and greens to bring some color to the surroundings and give the page some life. With this color scheme, the two monuments will be nicely accented and the page will look vibrant and alive!