"proves a horse of a different colour"

'Horse of a different colour' is an expression meaning that an idea or plan has to change, because of new information or developments.  It is believed that the phrase originated in horseracing, where the colour of the winning horse may be different from the horse one had backed to win.  It has appeared regularly in literature throughout the centuries; at one point in Shakespeare's 'Twelfth Night', Maria says "My purpose is, indeed, a horse of that colour".  Within popular culture, it is well known for its quite literal appearance in 'The Wizard of Oz'.