The three spirits each represent the message they are trying to send. Each ghost is a visual metaphor for its time. Marley, for example, is fettered. The chains are the ones he forged in life, by not looking out for mankind.
“I wear the chain I forged in life…. I made it link by link, and yard by yard; I girded it on of my own free will, and of my own free will I wore it….”(Stave 1, p. 14)
The Ghost of Christmas past is neither young nor old, demonstrating the fleeting and changing nature of memories.
It was a strange figure—like a child: yet not so like a child as like an old man, viewed through some supernatural medium, which gave him the appearance of having receded from the view... (Stave 2, p. 18)
The figure is like a man and like a child, and is both there and not there—just like memories. Our memory is fleeing and changing and not quite clear.
The Ghost of Christmas Present is boisterous and generous, demonstrating the Christmas Spirit Dickens wants us all to embody.
It was clothed in one simple deep green robe, or mantle, bordered with white fur. This garment hung so loosely on the figure, that its capacious breast was bare, as if disdaining to be warded or concealed by any artifice. (Stave 3, p. 29)
The Ghost of Christmas Present has his chest uncovered, Dickens implies, because he has an open heart. His entire demeanor and appearance are supportive and positive, except near the end when he shows Scrooge the two wretched children Ignorance and Want.
The Ghost of Christmas Yet to come is dark, and unspecific. We don’t know exactly what the future holds, and the future is a little scary!
It was shrouded in a deep black garment, which concealed its head, its face, its form, and left nothing of it visible save one outstretched hand. (Stave 4, p. 41)
Since there are many unknowns in the future, we do not see much of this ghost. He looks kind of nondescript, almost death itself. He does not talk, he only points. Scrooge is afraid of him, as we often fear the future.
See etext pdf for pages: http://www.enotes.com/christmas-carol-text