setColor and fillRect
The method setColor determines the color used for following operation (such as fillRect). setColor takes a color as a parameter.
You can use one of the stadard colors (such as Color.red) or you can use a custom color using the constructor new Color(10,20,30) which uses red, green and blue components 10, 20, 30. Each component can be a value from 0 to 255.
The method fillRect(x,y,w,h) takes four parameters x,y (for the top left corner) and w,h for the width and height of the rectangle to be filled.