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)

Metebelis III was the famous blue planet of the Acteon Galaxy. (TV: Carnival of Monsters, The Green Death)

The sun briefly turned blue when the Slitheen began draining its power. (TV: Revenge of the Slitheen)

Humanoid Daleks, Vogans, Monstrons and Zerovians had blue skin.