In administration panel go to Configuration => Configure modules
Select Availability module
Stockroom quantity (from) is the minimum number of available products. According to this value, products quantity ranges are defined for all availabilities.
Color is displayed if an availability doesn't have it's image.
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Select Configuration
In administration panel go to Offer => Products
You can set an availability manually or select Set according to the stockroom quantity than an availability is automatically adjusted according to a product's stock.
Check out stockroom documentation.
You can use import / export feature to assign an availability to the selected products.
Check out products documentation.