This paper investigates the color rank system of the Wonsam, ceremonial topcoat, worn as a court formal costume of the Joseon dynasty by analyzing the court costume system and the actual examples of wearing. The research shows that there were some discrepancies of the color rank of the Wonsam between the court costume system and the actual wearing examples. There were red, purple, blue, green, and black Wonsam in the Joseon dynasty. The color rank of the Wonsam is as follows: the Queen's color was red, the Crown Princess's was green and purple, the royal concubine's was usually green and blue, but they could wear purple when they won the King's favor. A prince's wife's was usually green, but she wore blue and purple if she became the mother of the King. The princess's was green, and the court lady's was green, blue, and black. In most cases, the textiles of Wonsam were made by silk with patterns, even though Joseon dynasty was ordered to use silks without patterns in court weddings and funeral ceremonies to avoid extravagance.