Source : Webster's Revised Unabridged Dictionary (1913)

Congo red \Congo red\ (Chem.)
   An artificial red dye from which the Congo group received its
   name. It is also widely used either in aqueous solution or as
   test paper (

   {Congo paper}) for the detection of free acid, which turns it
      blue.

Source : WordNet®

congo red
     n : a red-brown azo dye especially as a chemical pH indicator
         (congo red is red in basic and blue in acidic solutions)