false colours

pl n
  1. a flag to which one is not entitled, flown esp in order to deceivethe ship was sailing under false colours
  2. an assumed or misleading name or guiseto trade under false colours
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012