TARDIS blue

 was a shade of blue that was used on the exterior of the Doctor's TARDIS. Both Amy Pond and the Eleventh Doctor characterised the envelopes an older version of himself had sent as "TARDIS blue". (TV: The Impossible Astronaut, Day of the Moon)

Behind the scenes
"TARDIS blue" is a shade of blue with highly variable definition. As used in Day of the Moon, it clearly means "the colour of the Eleventh Doctor's TARDIS". However, as the TARDIS has been several different shades of blue over time, "TARDIS blue" is likely a relative term more than a specific one.