Stampin' Up! retires colours based on sales history and focus group feedback. They also look at overall colour offering and determine what colours are not working with others. They sent our surveys, looked at trends, and determined what colours are out of date, etc. With the changes, Stampin' Up! found that a few colours fit better in a different collection. They also organized the colours to be more complementary within each collection.
Check out this great video from Stampin' Up! that explains a bit more about the colour refresh: