Blue

Blue was the official colour of mourning on Necros. (TV: Revelation of the Daleks) It was also associated with the Doctor's TARDIS, as it was in the shape of a Police box. When the Eleventh Doctor sent a blue envelope to Amy Pond and Rory Williams, Amy knew he was the one who sent it, recognising the colour as "TARDIS blue". (TV: The Impossible Astronaut)