The scene is a whimsical one - a large, grand castle-shaped cake stands at the center of the room, commanding attention. Its fondant is a soft pink, fading to a light, sparkling gold at the edges of the turrets. The trees and bushes surrounding the cake are green, with pops of vibrant yellows and oranges for berries. Lastly, the banner hovering above the cake exclaims "Happy Birthday Harry Potter!" in bright, bold letters, adding a hint of adventure and magic to the page. To bring the page to life, I would use a light beige and baby blue palette to fill in the room, and then utilize a mix of soft and bright colors to shade the cake and its decorations. Lastly, the banner would be done in a crisp, navy blue for contrast and emphasis.