and here's what Wikipedia says:
"Reid told Songfacts that he got the title at a party, which gave him a starting point for the song. He overheard someone at the party saying to a woman, "You've turned a whiter shade of pale," and the phrase stuck in his mind. The original lyrics had four verses, of which only two are heard on the original recording. The third verse has been heard in live performances by Procol Harum, and more seldom also the fourth. The author of Procol Harum: beyond the pale, Claes Johansen, suggests that the song "deals in metaphorical form with a male/female relationship which after some negotiation ends in a sexual act." This is supported by Tim de Lisle in Lives of the Great Songs, who remarks that the lyrics concern a drunken seduction, which is described through references to sex as a form of travel, usually nautical, using mythical and literary journeys. Other observers have also commented that the lyrics concern a sexual relationship.
The phrase a whiter shade of pale by Keith Reid has since gained widespread use in the English language, noticed by several dictionaries.As such, the phrase is today often used in contexts independent of any consideration of the song. (See for many annotated examples complete with links to original sources.) It has also been heavily paraphrased, in forms like an Xer shade of Y - this to the extent that it has been officially recognized as a snowclone - a type of cliché and phrasal template."