Which of the following factors does not directly influence a vehicle's horsepower?

Vehicle color does not have any impact on the horsepower a vehicle generates. Horsepower is fundamentally a measure of the engine's power output, which is influenced by various mechanical factors related to engine design and performance enhancements. Engine size plays a direct role, as larger engines typically can displace more air and fuel, leading to increased power. Turbocharging enhances engine efficiency and power by forcing more air into the combustion chamber, thus increasing the horsepower. Fuel type influences how effectively an engine can perform, as different fuels can produce varying levels of energy and combustion efficiency. In contrast, the color of the vehicle is purely aesthetic and does not contribute to the engine's performance metrics.