If you expand by a certain property (e.g. "color"), only those property options should be preselected in the listing article. If there are other properties (e.g. "Size") in addition to the color, all expanded items will automatically have an additional size preselected. However, it would be better to pre-select only the "Color" and to display a "Details" button in the listing instead of the "Add to cart" button, so that the user has to select the - in this case "Size" - himself before adding the item to the cart.