Why is the de-icing fluid sprayed on jets coloured orange?
There are actually two types of de-icing fluid, one is orange and one is green. According to a product safety sheet for one brand:


  The dye is intended to indicate which parts of the  aircraft have been
  treated and to differentiate between fluids (Type I fluids are orange,
  Type IV  fluids are green).


As to why orange and green, presumably they aren't easily confused with other liquids used around aircraft. Black would look like oil or another fluid if it pooled on tarmac, hydraulic fluids or oils are often yellow/green in colour and blue is a very common colour in aircraft painted liveries. So I guess that orange and green were sort of the ones 'left over' after all the other obvious colours had been considered.