Flash MX 2004.
fill.posArray
Property; an array of integers, each in the range of zero to 255, indicating the position of the corresponding color. This property is available only if the value of the fill.style property is either "radialGradient" or "linearGradient".
The following example specifies the colors to use in a linear gradient for the current selection:
var fill = fl.getDocumentDOM().getCustomFill();
fill.style = "linearGradient";
fill.colorArray = [ 0x00ff00, 0xff0000, 0x0000ff ];
fill.posArray= [0,100, 200];
fl.getDocumentDOM().setCustomFill( fill );