Why may an object appear to have less volume when painted with a single, flat red-orange colour?

Correct answer: There is no variation in value, temperature, or saturation

Changes in value from light to shadow, and sometimes shifts in colour temperature or saturation, help show how a solid surface turns. A uniform red-orange provides little such information, so the object may look flatter. Red-orange is a valid colour, and line alone is not the only way to suggest volume.