In our solar system, there are two blue dots: Earth and Neptune. This leads to the question - suppose an early Earth-like planet has a methane-dominated atmosphere. How could you tell such a planet from a Neptune, or a Titan? The first approach is simply to use celestial mechanics to give the mass and stellar separation of the objects in question. Large mass tells you it's a giant. Large separation tells you it's probably too cold, but spectroscopy can give you the temperature and prove it. Is that true? Not always. Spectroscopy can also identify abundant H₂ in a planet's atmosphere, again proving it's a giant. What about Titan? IR observations alone don't give you the surface temperature. Hence you can't rule out an intense atmospheric greenhouse effect that could raise the surface temperature into the liquid ammonia-water range even at large separation of object and star. Such models existed for Titan before radio wavelength observations showed the surface was very cold, hence atmosphere was thinner than those models required (and did not contain NH₃).