That green light on SEO for WP by Yoast plugin is driving me crazy! Does it have to be green?

No! Those are suggestions for placement of keywords. The spirit of the ‘law’ applies here. Make the topic clear to your readers, add keywords where appropriate. By doing this, you will have orange and green lights. If your light is ‘orange’ it means you should take another look and be sure that the topic is clear. If it is -you’re golden!