Different node types are colored differently for easy identification. For example, animation nodes are a light blue, joints are green, and lights are beige.
Select Window > Settings/Preferences > Color Settings. Under the General tab, expand the Node Editor section, then expand the Node Types section for the list of default colors used for each node type. You can customize the colors for each node type by clicking on the color swatch to select a color using the Color Chooser.
