When the key is turned to the first, or RUN position, the "red" oil pressure light comes on. The second position, or first spring loaded position is the glow plug ON position, and the "amber" indicator light comes on. The third position, or second spring loaded position is the START position. Once the engine starts and the key is allowed to return the first or RUN position, both the "RED" and "AMBER" lights go out. All three of your Kubota's are started in the same way. The Grand L tractors uses a timer on the glow plugs that shuts of after a few seconds, but the BX, B, RTV, and F series all must be manually pre heated by holding the key in the first spring loaded position.
Depending on how cold it is depends on how long it is necessary to hold it on. As a general rule, I advise my customers to hold the key and count to about 10. You may need to hold it longer if it is still needs to be spun more than three or four times before it starts.
Hope this helps.