Changing masks in multiple groups to single colour.

Based on your original test file, I have a working script... However, it stops when it hits a top-level group that doesn't contain the required sub-group and colour fill (i.e. Group 3). If you remove Group 3 or move it to the bottom of the top-level layer stack, then all other top-level layer groups will be processed correctly. I don't have time at the moment to make it skip over groups that don't meet the required conditions, but perhaps somebody else could modify the code or post an alternative script.

// Set the layer groups variable
var theLayerSets = activeDocument.layerSets;
// Forward loop over the layer groups
for (var i = 0; i < theLayerSets.length; i++) {
    // Check if the layer is a group and a top level group
    if (theLayerSets[i].typename === "LayerSet" && theLayerSets[i].parent == activeDocument) {
        // Select the top layer in the "Cushion Blanket Colour" group
        activeDocument.activeLayer = theLayerSets[i].layers["Cushion Blanket Colour"].layers[0];
        // Change the colour fill
        setColorFill(0, 0, 255);
    }
}

function setColorFill(red, grain, blue) {
	function s2t(s) {
        return app.stringIDToTypeID(s);
    }
	var descriptor = new ActionDescriptor();
	var descriptor2 = new ActionDescriptor();
	var descriptor3 = new ActionDescriptor();
	var reference = new ActionReference();
	reference.putEnumerated( s2t( "contentLayer" ), s2t( "ordinal" ), s2t( "targetEnum" ));
	descriptor.putReference( s2t( "null" ), reference );
	descriptor3.putDouble( s2t( "red" ), red );
	descriptor3.putDouble( s2t( "grain" ), grain );
	descriptor3.putDouble( s2t( "blue" ), blue );
	descriptor2.putObject( s2t( "color" ), s2t( "RGBColor" ), descriptor3 );
	descriptor.putObject( s2t( "to" ), s2t( "solidColorLayer" ), descriptor2 );
	executeAction( s2t( "set" ), descriptor, DialogModes.NO );
}

https://prepression.blogspot.com/2017/11/downloading-and-installing-adobe-scripts.html