Final answer:
The question pertains to a plot or character decision in a literary context, which without specific text reference, can only be speculated on as a device to introduce surprise or character development.
Step-by-step explanation:
The question about why Annie's mom and dad did not tell Annie and Ted the house was purple appears to be a narrative element that would be found in an English literature context, particularly one involving analysis or comprehension of a story's plot or character actions.
Without further context or reference to a specific text where this scenario occurs, it's impossible to provide a definitive answer. However, it's likely that the choice not to inform the children about the color of the house may have been a narrative decision by the author to introduce elements of surprise, mystery, or to develop the characters' personalities and family dynamics.