Create attribute Color&size.
1. Create a parent attribute
Create main attribute Color&Size without any values. Element type: - selectbox.
2. Then create a new child attribute Size. Set Attribute Parent: Color&Size.
Element type: - selectbox. Here you need to add option values like XS, S, L...
3. Then create new child attribute Color. Set Attribute Parent: Color&Size.
Add option values like white, black, red..etc. Element type: - selectbox.
4. Add Option to Product
Now if you go to product edit page click to add a new Color&Size global attribute to the product.
5. Add the Option's values to the Product
Then add option value like black:XS black:S white:L white:XS red:S red:L or any other combination of product you have to sell.
In the storefront product page, the customers can choose combined option value
