The color of the quantity box indicates whether it is out of stock or less than the inventory warning level you’ve entered. The colors may vary depending on the stock level, quantity level and your inventory level settings.
If your product is not out of stock and you didn’t set an inventory level warning, the quantity box will be white; this is the default setting.
If your product is out of stock, the color of the quantity box will be red. This means that you have zero quantity for this product.
If your product has an inventory warning level number entered, the quantity box will turn yellow when the current stock level is less than the inventory warning number that you used for the product. This is a really simple and visual way to understand if you’re running out of stock, and make sure you add additional quantities for the product if you can.
You can set an inventory level warning number under the “General>Quantity” tab after clicking edit the product.