That’s a given. But by casting an actual vote for “green party”, you are not only electing green party, but you are also sending strong signal to other parties

That’s a given. But by casting an actual vote for “green party”, you are not only electing green party, but you are also sending strong signal to other parties, that there is an active voter who cares for “green” ideas. (Other parties may opt to include some “green” green ideas into their campaign, or even schedule)