Near the beginning of chapter 1, there is a hermit who lives in a hut on a beach, and who amongst other things keeps a fire burining constantly. Now this fire is described as fuelled with driftwood (washed up on the shore), and burning with a blue flame.
In reality, driftwood is saturated with salt (it's been in the sea, after all), so as anyone knows who knows basic chemistry, it burns with a bright yellow flame...