Using 'why noun' instead of 'why noun?' in a title? [closed]

Solution 1:

There is considerable leeway given to titles, as evidenced by terms such as headlinese. Quite separately, however, “why green” can be parsed as a question, or alternatively, as the introduction to an answer.

As a question, it can be thought of as an ellipsed form of “Why is it green?”

As a non-question, it can be thought of as an ellipsed form of “This is why it is green:”.

They can be distinguished in spoken form by the relatively common convention of raising the pitch at the end of a question and lowering it at the end of a statement.

You asked whether the versions with and without a question mark are equivalent. They aren’t, since the version without a question mark might not be a question. However, since the text is in printed form and since it introduces the answer, there is no practical difference in this case.