Hi Gianpiero,
When adding a new products, at “Product Data” please select “Variable product” https://i.imgur.com/ZtmnN0t.png
So you will have the possibility to add different attributes to the product. The attributes must be created first on the “Attributes” tab. https://i.imgur.com/orqisiU.png
You can add as attributes colour, size, etc.
Then, because you have selected “Variable product”, when adding the new product at the “Product Data” box you will have a tab called “Variations”. And there you can add as many variations as you want.
For example:
BAG (SKU: 123bag) > green > small > 10 qty
BAG (SKU: 123bag) > green > large > 15 qty
BAG (SKU: 123bag) > violet > small > 50 qty
You can have even different SKU’s for each every variation.
Links to read:
https://docs.woothemes.com/document/managing-products/
https://docs.woothemes.com/document/product-variations/#section-2
https://docs.woothemes.com/document/managing-products/
Hope this helps you.
Regards,
Miklós