The answer is assimilation.
According to Piaget's theory of cognitive development, children go through a stage of development known as assimilation. Assimilation involves adapting to new experiences and information by fitting it into existing knowledge, perceptions and schemas. In this instance, Darlene's error in mistaking the vitamins for candies is likely caused due to her 'assimilating' the appearance of vitamins with her existing knowledge of what candies look like.
