There is one man with light blue jeans on, and one with dark blue jeans on that are walking down the street.
Two men are wearing jeans for a jean advertisement.
The men are wearing blue jeans and walking down the street because they are in a jean advertisement.
If the is one main with light blue jeans on and one with dark blue, there are two men wearing jeans.
Two men are wearing jeans because one is with light blue jeans on and one is with dark blue jeans on.