In a certain code '642' means 'sky is blue', '52' means 'is calm' and '417' means 'blue and beautiful'. What number will indicate the word 'blue'?

Let's compare the code sentences:
1. '642' $\to$ 'sky is blue'
2. '52' $\to$ 'is calm'
3. '417' $\to$ 'blue and beautiful'

Comparing (1) and (2), the common digit is '2' and the common word is 'is'. Hence, '2' means 'is'.
Comparing (1) and (3), the common digit is '4' and the common word is 'blue'. Hence, '4' means 'blue'.