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Two main reasons1) the food colouring was probably less volatile than the water so it stayed in the pan and didn't evaporate
2) Steam is strongly white because it is made up of lots of tiny droplets and you get reflections and refractions very strongly at each air - water interface, these reflections and refractions are not dependent on the colour of the liquid in the drop. They are so strong that most light that is reflected back out probably only goes through a very few droplets, and as the droplets are very small it passes through very little actual water. You would need an awful lot of food colouring to make <.5mm of water look strongly coloured.
Steam is strongly white