The ink's color does not come back unless it is subjected to temperatures below 14°F. There is one thing that I would warn you about, not all of the colors work well. Some of the colors will be visible, although very faint, after ironing. White fabric, being the hardest to hide something on, was what I used to test the three Frixion pens I bought. The colors in my set are purple, orange and pink. The purple vanishes very well. The orange leaves some trace color and the pink functioned the worst. The pink was still 'noticeable' after heating.
They all come back if they are taken down below 14°F, as they should. Reheating results in the same outcome as above.