As you're creating a shipment or a sales order, you may come across the quantity field either highlighted red or yellow. Below is what each of those mean.
Yellow Highlighted Quantity
This means that the quantity on the order or shipment will make your available quantity negative, but NOT your QoH. When would this commonly happen? If you have 10 in stock, but already have a reservation for 5 units that are unshipped, creating a new order for 6 will make the available stock -1. Why doesn't this make it red? Because it hasn't been shipped yet, meaning the QoH has not yet been adjusted.
Red Highlighted Quantity
This means that the quantity on the order or shipment will make your QoH (Quantity on Hand) negative. Using the example above, 15 is greater than the 10 we have in stock, so the field is highlighted red.
One thing to note, is this field will ALWAYS be based off of real time stock levels. If you are viewing an older order that had stock available when it was shipped, but it no longer does, then the field will be red. The highlighted fields are primarily used when creating shipments and orders as a stock warning.