You can change the transparency of the color you assign different graphics.
var trans = Math.random();
// lineSymbol used for freehand polyline, polyline and line.
var lineSymbol = new CartographicLineSymbol(
CartographicLineSymbol.STYLE_SOLID,
new Color([255, 0, 0, trans]), 10,
CartographicLineSymbol.CAP_ROUND,
CartographicLineSymbol.JOIN_MITER, 5
);