Line color is determined by the water type you select in the component box. There are four colors indicating water type: red for hot, blue for cold, green for non-potable, and gray for unspecified. To change the color, from the component box, right click the hamburger menu, and click Edit component to access to data fields form for the component.
LegiTip: Line color defaults to the component type i.e. tank heater defaults to a red line (hot water), incoming cold water defaults to a blue line (cold water).