matteColor

var savefile=new File("~/Desktop/exp.gif");
var exportOptions = new ExportOptionsGIF();
exportOptions.matte = true;
exportOptions.matteColor = myColor();
app.activeDocument.exportFile(savefile, ExportType.GIF, exportOptions);
function myColor(){
    var color = new RGBColor();
    color.red=255;
    color.green=0;
    color.blue=0;
    return color;
}