When did the Otis family take serious notice of the ghost?

The continual renewal of the blood stain on the library floor excited the attention of the Otis family. It was a strange occurrence considering the fact that the door was always locked at night by Mr Otis and the windows were kept closely barred. The chameleon-like changing of colours attracted a good deal of comment. Some mornings it was a dull red, then it would be vermilion, a rich purple and once when they came down for family prayers, they found it to be bright emerald-green. These kaleidoscopic changes amused the party very much and bets were freely made on the subject every evening. This unexplained occurrence made them believe that no rational explanation could be offered to understand this.