Sky blue was a light form of blue so called after the colour of the sky.
Diane Holmes wore a sky blue shirt, with her sleeves rolled up, when adapting to the 21st century. (TV: Out of Time)
Sarah Jane Smith sometimes wore a sky blue shirt. (TV: The Day of the Clown, Enemy of the Bane)
Tegan Jovanka wore a sky blue shirt during the Master's Dalek Plan, (TV: The Power of the Doctor) when helping many versions of the Doctor and companions, (GAME: Lost in Time) and when reuniting with the Fifth Doctor in the Memory TARDIS. (TV: Earthshock)
The Tenth Doctor wore a sky blue shirt. (TV: The Idiot's Lantern) He also wore a tie in ribbed umber brown, with a white and sky-blue repeating stylised flora design in diagonal bands when he meets Professor Yana (a human form of the War Master) and the Malmooth Chantho on the year 100 trillion AD, (TV: Utopia) and when dealing with the Saxon Master and the Toclafane during their invasion of Earth. (TV: The Sound of Drums|Last of the Time Lords)
The Eleventh Doctor sometimes wore a sky blue shirt , (TV: The Bells of Saint John, The Day of the Doctor, et alt.) leading up to his regeneration into the Twelfth Doctor (who consequently spent his first hours in that same shirt). (TV: The Time of the Doctor)
Rachel Jensen wore a sky blue jumper with a turtleneck collar when she first met the Seventh Doctor during the Hand of Omega Incident. (TV: Remembrance of the Daleks)
The Third Doctor wore a sky blue shirt up to his death. (TV: Planet of the Spiders) His successor, the Fourth Doctor, wore it briefly. (TV: Planet of the Spiders, Robot)
Lieutenant Harry Sullivan, a doctor from UNIT, wore a sky blue shirt when travelling with the Fourth Doctor. (TV: Robot, The Ark in Space, The Sontaran Experiment, Genesis of the Daleks, Revenge of the Cybermen, Terror of the Zygons)
Bilis Manger wore a sky blue shirt. (TV: Captain Jack Harkness, End of Days)
Joseph Chambers, the Cabinet minister entrusted with the safety of the destructor codes for all the world's nuclear missiles, wore a sky blue shirt when the K1 Robot killed him on orders from the Scientific Reform Society. (TV: Robot)
Kate Stewart wore a sky blue shirt during the 3W Institute Affair. (TV: Death in Heaven)
The General's uniform during the original Gallifreyan Civil War was sky blue, complemented by golden epaulettes. (PROSE: Warmonger)