The point of a cover is to do the exact same job of the book – to emotionally connect with readers. In case of a cover, that connection has to happen within a fraction of a second – a reader looking at a thumbnail view of a cover probably won’t notice if a heroine’s hair is light blonde vs honey blonde, but they will care if they see a light airy cover and are in a mood for a dark angsty tale. (I do love dark angsty tales – someone once referred to it as romangst – how cool is that?)
Actually scratch that – readers won’t care. They’ll simply pass over the book and look for something that visually appeals to whatever they are in the mood for. So aside from characters and symbolism and visual keywords, the best thing you can do for your cover artist – and your bottom line – is to give a great visual description of the emotional impact of your story.