In a certain code language, 591 means 'sky is blue', 256 means 'sea is deep', and 609 means 'sea looks blue'. Which number is the code for 'blue' in that language?

9

Given; In a certain code language, 591 means 'sky is blue', 256 means 'sea is deep', and 609 means 'sea looks blue'.

So,

5   9   1  =  sky  is  blue

2   5   6  =  sea  is  deep

6   0   9  =  sea  looks  blue

Thus,

Code for sky is 6, 'Is' is 5, sky is 1, deep is 2 and code for blue is 9