When you feel you are sufficiently adept at using the other Elements, you may begin to use spirit – the fifth Element.
This has no special space but is everywhere.
It should never ever be used negatively because, particularly when you are weak and tired, it can rebound on you.
You may well find that you instinctively link strongly with the life force as spirit, in which case you are probably succeeding in bringing all the Elements together within yourself.
There is no particular colour associated with spirit – perception is all-important.
If you choose to represent spirit on the altar, you may do so however you wish.
You are free to use your intuition and you must have a very strong awareness of your reason for choosing that particular symbol.