In a certain code, '467' means 'leaves are green', '485' means 'green is good', and '539' means 'they are playing'. Which digit stands for 'leaves' in that code?

The digit 7 stands for 'leaves'.

The code '467' is 'leaves are green' and '485' is 'green is good'.

The digit common to both is 4, so 'green' = 4.

Removing 4 from '467' leaves 6 and 7 for 'leaves' and 'are'.

Matching the remaining word placement, 'leaves' corresponds to 7.