Blue
From Tardis Wiki, the free Doctor Who reference
Blue was the official colour of mourning on Necros. (DW: 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". (DW: The Impossible Astronaut)
