The underground map could be used as inspiration for a colour theme with a maps key giving the students name and allocating them with with a colour. A line could then be taped onto the floor to lead to thier work. The background of their work would also be colour coded.
Paint pots could be left to dry and made to look like the lines had been painted or just ran out of the pots.
Tube stations and slogans could also be used to continue the theming.
Another theme could be shapes......
Maybe a large cube supended from the ceiling with images printed onto each side? A large sphere could have images projected onto it distorting the images into intersting shapes.
Rich suggested an abstract design, that also be quite mind altering... Instead of just decorating the walls, you could do the ceiling and floor?