All ports and connection lines are represented by different colors to denote the attribute type for the input or output, for example, integer, string, boolean and so forth. The connection line takes on the color of the output.
You can customize the colors for different port types by navigating to Windows > Settings/Preferences > Color Settings. Under the General tab, expand the Node Editor section, then expand the Attribute Types section. Click on the color swatch beside each attribute type to select a color using the Color Chooser.
You can customize the default color for unconnected master ports by navigating to Windows > Settings/Preferences > Color Settings. Under the General tab, expand the Node Editor section, then click on the color swatch beside Unconnected Master Ports to select a color using the Color Chooser.