You want to specify the appearance of a line you wish to draw.

Use the lineStyle() method of the Graphics class to define line attributes.

The last line of the following code block sets any lines subsequently drawn into the display object sp to be of 1-pixel thickness and blue in color. The line settings for the Graphics object g remain in effect until changed or reset.

var sp:Sprite = new Sprite();
addChild(sp);
var g:Graphics = sp.graphics;
g.lineStyle(1, 0x0000FF);

The first parameter is thickness, a pixel value, with 0 being a hairline. The second parameter is the color of the line.

You also have additional optional parameters that closely mimic the Property inspector settings. These parameters include alpha, pixelHinting (also called stroke hinting, a Boolean to determine if the lines are drawn on the whole pixel), scaleMode (to determine if and how lines are scaled when the parent container is scaled), caps (specifying type of line end cap used), joints (specifying type of joint type used), and miterLimit (determining how sharp corners appear).

If you want to clear everything previously drawn into the display object, you can use the clear() method of the Graphics class. However, because a line isn’t drawn when a pixel thickness isn’t specified, you can also clear just the line attributes by passing no parameters to the lineStyle() method.

g.lineStyle();

the section called “12.6 Defining a Fill Style” for a demonstration of using multiple lineTo() commands to create a triangle.