To change the fill of a symbol, click on it, or several symbols (How?), then click Fill in the Shape Style group of the Home tab and choose from the menu.

Not all library symbols can be filled properly due to the nature of the symbol, but we recommend you try changing the fill and line color of a symbol to see how it handles these actions. You can always undo! (How?)
To change the color of the border of a symbol or the color of a line, select the symbol or line and then click Line in the Shape Style group of the Home tab (next to the Fill button). This menu also contains menus for choosing the thickness and dashing pattern of the selected line(s).
How to Add Effects >>