If the product you have chosen is available in many colors and sizes, first of all choose the color you want. Then, you will find out the sizes that are available in that color. If one color is not available, it will be crossed and you won’t be able to click on it. If the product you have chosen is available in one color or one size, you can select it and add it to your shopping bag. If it is not available, « Stock shortage » will show up.