Le bleu perdu.

An animated cartoon envisaging the kind of world that children of the future may well inherit when the last vestige of blue is blotted from the sky by the spreading mantle of smog. In this story a little boy sets out, equipped with celestial wings, to discover whether what an old man has told him is actually true, that there is blue above the grey. When he finds it he concludes that blue is where paradise is; the grimy earth is the netherworld.