That binding, new go.Binding('stroke', '')
means that you are trying to set the Shape.stroke property to the whole value of the item in the Panel.itemArray.
But in the example that I posted above, that Array holds the text strings to be shown. It seems unlikely that those text strings are CSS colors that are valid values for the Shape.stroke.
So I suggest that before you show the Adornment by adding it to the Node, that you modify the itemTemplate’s shape’s stroke to be the color that you want all of those rounded rectangles to be, rather than the “purple” it is hardcoded to be.