A third color appears when two overlapping colors mix together and visually create a new one. This isn’t always a mistake as designers often intentionally use overlapping inks to create new shades, especially in techniques like screen printing. But when it’s unintentional, it can throw off your entire design. Usually, an unintential color is a result of trapping or misregistration. To hide gaps between colors, printers add a bit of overlap (trapping), but when those overlapping colors are semi-transparent or not carefully managed, they mix and create a third color that wasn’t meant to be there. An example of this would be a bright yellow trapped into a rich blue creating green around the edges.
Unintentional third colors can change the look of your design, distract from the layout, and dilute brand colors if you are using specific inks or Pantones.