TARDIS blue

 was a shade of blue that was used on the exterior of the Doctor's TARDIS. Because the shell of the TARDIS was based on a Metropolitan Police Service police box, the blue might also have been thought of as "police blue". The Eleventh Doctor once characterised the envelopes an older version of himself had sent to Rory, Amy, Canton, River and himself as "TARDIS blue". (TV: 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.