If the turtle's pen state is PENDOWN, FILL colors the area with the current pen color. If the pen state is PENERASE, FILL colors the area with the current background color, thus erasing the form. If the pen state is PENREVERSE, FILL colors the area with the complement of the current area color; for example, white turns to black, and green to red. If the turtle's pen state is PENUP, FILL has no effect.

If the penwidth is set to 1 and the turtle is on a horizontal or vertical line, FILL changes the color of the line and other connected vertical and horizontal lines to the current pen color. If the line is at any other angle, FILL has no effect.