HOW can i change the colour of mercury to red safely?

Answer 1

Mercury is a metal that just happens to be liquid at room temperature. I don't know of any dye that would dissolve in it. It will dissolve other metals but the color seems to stay pretty much silvery. If you found a finely divided, red-colored inorganic pigment that was close to the same density as mercury, it might stay in suspension long enough to be usefull, but I doubt there are any pigments dense enough to work.

If you are experimenting with mercury, please be careful; spills are very difficult to clean up and most of it's compounds are more or less toxic.

If you are trying to make thermometers, the ones with the red liquid are usualy filled with alcohol (dyed red, so it shows up easier).