3 links have been added on this idea.
  1. You can already get ink that changes colour depending on the heat, I guess they ain’t got round to applying it to work over a period of time. But maybe they should. This would be handy for all sorts of things, Like just writing a line on anything that you don’t want to go over a certain temperature – and seeing the ink if it has. Lots of applications tbh. nice idea.

  2. Like the idea of just drawing a line on the egg and waiting till it changes to the right colour, but for those who can’t wait for that to be invented, you could always pop one of these in the pan…

    “Heat sensitive color changing indicator signals Perfect for cooking multiple eggs at the same time”