To edit Vehicle Registration Warning Intervals, click on your username at the top-right of the screen (1) and then click Settings (2):
On the left-hand side of the screen, use the scroll bar (1) to scroll to the bottom of the list. Once you have reached the bottom you will see Vehicle Settings (2), click on Vehicle Settings:
You can then modify Vehicle Settings by clicking on values in the Value column (1). The values will turn blue when you place the mouse over them. You can also create your own labels by clicking on the Spare Fields in the Value column and namely them as you wish (2).
In the above example, registrations will turn RED when due in 14 days or less and YELLOW when due in 30 days or less. The red colour will always override the yellow colour.
Changes you make are automatically saved after leaving the editor (the button will no longer be blue).