This story plays on the perceived pragmatism of Americans, who were, in the late 19th century, using their newly growing wealth and power to buy their way into English life. Their pragmatism collided with English culture, perceived as steeped in history and tradition.
The Canterville ghost represents the history of ancient Canterville Hall. The Otises, quite American and practical, don't believe in him. He leaves a bloodstain on the library floor. The bloodstain is inconvenient, so they simply scrub it out. It returns, which raises doubts in their minds (maybe there is a ghost), but it is not until they actually see the ghost that they are convinced it is real. Even so, the ghost doesn't frighten them.
The ghost, determined to do his job of scaring the family, starts changing the color of the bloodstain. Sometimes it is more vermilion, sometimes more purple, and once emerald green. The family treats this as a great joke--except for Virginia. The changing colors distress Virginia and she almost cries the day it is emerald green.
Through the changing colors of the bloodstains, Virginia begins to intuit that there is more to the ghost than a mere joke. Perhaps she sees the different colors as reflecting the ghost's changing emotions or as a sign of his desperation. Whatever the case, she refuses to join the family in laughing at him.
The changing bloodstains thus begin to change Virginia's heart, and when she encounters the ghost, she treats him with compassion, launching the more sentimental story of love and compassion that lurks beneath the comic tale of a ghost unable to frighten a family. Virginia shows the human side of American pragmatism: by treating the ghost with kindness, she moves Canterville Hall to a new place, a place no longer haunted by the demons of the past.