Which factor is NOT known to influence the efficiency of a hydraulic pump?

The factor that does not influence the efficiency of a hydraulic pump is the color of the fluid. While the physical properties of the hydraulic fluid, such as viscosity, play a significant role in the pump's performance and efficiency, the color itself does not impact how well a pump operates. Viscosity affects the fluid's resistance to flow and can influence how easily a pump can move the fluid through the system, impacting efficiency. Additionally, different pump types and their operating conditions can greatly affect performance, further emphasizing that the color of the fluid is irrelevant in terms of efficiency.