Bertha's pinafore is described as 'spotlessly white' in 'The Story-Teller' to symbolize her innocence and purity.
In 'The Story-Teller' by Saki, Bertha's pinafore is described as 'spotlessly white' because it is a symbol of her innocence as well as her pureness. The spotless white color represents her purity and lack of blemish, emphasizing her innocence in the story.