CanvasModulate tints everything drawn in 2D with a color. For example, if you have a tilemap with some sprites and set a reddish color in CanvasModulate, your sprites and tilemap will become reddish. It is useful to adjust the color tone of a level, doing day/night effects etc.
You can set the tint color by clicking on the CanvasModulate node, then modify the color property in the inspector dock.
if you want to modify this color using a script (like changing color if something happens in the game), that’s where you would use get_color and set_color. Like this: