In substitution type of questions, some particular objects are assigned code names. Then a question is asked that is to be answered in the code language.
If water is called blue, blue is called red, red is called white, white is called sky, sky is called rain, rain is called green and green is called air, which of the following is the colour of milk ?
The colour of milk is 'white' and as given 'white' is called 'sky'.
So, the colour of milk is 'sky'.