what is the song yellow from coldplay about
A reference to Stephanie's "yellow glow"
Yellow (Coldplay song)
Martin later titled the song "Yellow" as a reference to Stephanie's "yellow glow". 
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Yellow (Coldplay song) [SEP] Background and inspiration
He was thinking of a specific word, which he deemed a missing keyword in the lyrics, to fit the song's concept. He looked around the studio and saw Stephanie, a friend who happened to be present in the studio at that time. Martin later titled the song "Yellow" as a reference to Stephanie's "yellow glow". The lyrics progressed from there, with the band collaborating. Bassist Guy Berryman came up with the opening line "Look at the stars". That night, having quickly composed the song, the band recorded it. A much different origin of the song was told on "The Howard Stern Show" in November 2011.