At the top right of the orders screen you will find the labels menu. This is where you can define custom labels for categorising your orders.
Clicking the labels menu reveals a "And Label" dialog box. Provide the name of the label and click the And Label button.
When the label is added it appears as it have between Inbox and Archive.
It is possible to change the name or colour of an existing label by clicking the labels menu to reveal the list of labels then clicking the little gear icon to the right of the label, which reveals the label editor.
In the label editor, click the colour box to reveal a colour picker. Choose a colour from the colour spectrum bar on the right, then click anywhere in the box on the left to choose the shade of that colour.
The label editor also lets you change the label name you provided when originally creating it.
Clicking the delete button in the label editor, deletes the label and removes it from any orders that have been tagged with that label.
Orders can be tagged with a label from the orders screen or from the order details screen.
From the Orders screen
To take one or more orders from the orders screen, tick the checkbox in the last column on the order list for the orders you want to tag then click the labels menu and click on the labels you want to add or remove. Clicking a label toggles it on or off the selected orders.
From the Order Details screen
When you have an individual order open a panel showing the available labels is shown at the top right of the order screen. Tick any of the labels you want to tag this order with and remove the tick for any labels you no longer want on this order.
At the top of the orders screen you will see a tab for each label that has been defined. clicking the tab shows a list of all the orders that have been tagged that label.
In the left column of the order's list a small colour box for each label assigned to the order will be shown making it easy to see at a glance which orders have which labels.