Piping Input menu: Options > Pipe Density 
Legends toolbar: Pipe Density 
Static Output Processor menu: PlotOptions > Pipe Density
Static Output Processor Legends toolbar: Pipe Density
Updates the model to show each pipe density in a different color. Use this option to see the pipe density variations throughout the system or to verify that changes have been made. A color key displays the pipe densities defined in the model. You can change the assigned colors to meet your needs.