populating a Photoshop document with 256 colors from color table or swatch group

Thy this

try {

var w = 32;
var h = 32;
var gap = 2;

var idx = 1;

for (var y = 0; y < 16; y++)
    {
    for (var x = 0; x < 16; x++)
        {
        var x0 = x*(w+gap);
        var y0 = y*(h+gap);

        var x1 = x0 + w;
        var y1 = y0 + h;

        app.activeDocument.selection.select([[x0, y0], [x1, y0], [x1, y1], [x0, y1]], SelectionType.REPLACE);

        // select color by index
        var d = new ActionDescriptor();
        var r = new ActionReference();
        r.putIndex(stringIDToTypeID("colors"), idx);
        d.putReference(stringIDToTypeID("null"), r);
        executeAction(stringIDToTypeID("select"), d, DialogModes.NO);
        ++idx;

        // fill selection
        var d = new ActionDescriptor();
        d.putEnumerated(stringIDToTypeID("using"), stringIDToTypeID("fillContents"), stringIDToTypeID("foregroundColor"));
        d.putUnitDouble(stringIDToTypeID("opacity"), stringIDToTypeID("percentUnit"), 100);
        d.putEnumerated(stringIDToTypeID("mode"), stringIDToTypeID("blendMode"), stringIDToTypeID("normal"));
        executeAction(stringIDToTypeID("fill"), d, DialogModes.NO);

        app.activeDocument.selection.deselect();

        }
     }

} catch (e) { alert(e.line+ "\n\n" +e); }