In the later seasons of Little House on the Prairie, many people have questioned why Nellie Oleson was always dressed in a dark colored dress with a grey pinafore. For those who don’t want every episode in order, it becomes quite noticeable that she is always wearing a navy blue dress (although some mistake it for being a black dress).
When Nellie attended school in Winoka, this was the dress she wore as part of her school uniform, which was navy blue in color and with a lighter grey pinafore over top of it. Willie wore a uniform with a tie, which he commented how much he hated wearing it.
During the Winoka episodes, Laura and her family attended a makeshift school in a barn as they could not afford to attend the private school that Nellie and Willie attended.