I started a thread on here a few months ago about the Frixion pen and that I had tested it thoroughly; I thought I had.
Tuesday I marked some embroidery squares for machine embroidery on green, blue, and red. When you iron over the marks to remove them, they turn white. That's where the un-oh comes in. However, I took a wet cloth and removed it. Will it come back? I don't know.
Previously, all my testing was with a light piece of cloth. That's what the salesman at Houston used also. I never thought about the mark turning white on colors.