It's funny. To me, 'happy' songs are often the least so. They often seem vapid, lacking in personality or perhaps a type of sprirtiaulity. Lacking the essence of true human soul. I think melancholic songs can bring the most happiness. It allows you to truly connect with an artist, to feel you're not alone in the world. A truly crafted master piece of sad music doesn't leave you depressed, but rather opens you up and relinquishes your qualms. It puts you at ease and in a sense of bliss with the essence of shared human experience.
I know what you mean, but I would say that the appropriate song for your mood is the one that has the best ability to make you happy. It could well be that you require a dark sound and therefore, a dark song will actually make you happy. It could also be that you require a mixed CD comprised of songs to match the oceans tides, in which case I am well equipped :)