Neptune's blue color is largely the result of absorption of red light by methane in the atmosphere but there is some additional as-yet-unidentified chromophore which gives the clouds their rich blue tint.
The simplest molicule is Hydrogen. From what we know, that is what powers the stars. It is also one of the most prevalent molicules in the universe. It's all over the place. The methane on Neptune is not the result of Dinosaur farts.
Damn my spelin some times!