The deal with color-changing mugs is that, when hot liquid is added, a new picture magically (OK, magic is relative) appears. Heat-sensitive mugs are made with a special paint that takes advantage of thermochromism by swapping shades based on low and high temperatures.
When the temperature of the thermochromic mug is above 40 °C, the color changes. When the temperature of the cold color-changing mug is below 20 °C, the color changes and the watercolor is still not popular.