Silhouette: Red fills the upper half, White fills the lower half and button, and Black draws the equator that locks the circle together. Those three decisions are enough for instant recognition.
Volume: Light Coral lifts the red cap, Fire Brick turns its opposite edge away from the light, and Gainsboro keeps the white shell from looking flat.
Why it works: the added shades never compete with the central division. They describe a sphere while leaving the original three-color symbol intact.
