There have been many interpretations, but evidently it wasn't intended to be meaningful by the songwriter. "Martin went on to further explain that the word "yellow" has absolutely no meaning whatsoever and while writing the rest of the song he tried his best to change yellow to something else since every lyric before yellow made no sense but in the end the word "yellow" just sounded right. Martin also told Stern that through the years depending on the attitude and manner of whoever interviews him, he would make up some story about a song or album titles just to move on to the next question. Martin applauded Stern saying "I like you, Howard, so that's the first time I've ever told anyone the truth behind 'Yellow" http://www.thatericalper.com/2014/11/08/5-facts-you-didnt-know-about-coldplays-yellow/