Sky blue is a colour that resembles the colour of the unclouded sky at noon (azure) reflecting off a metallic surface.
The entry for "sky-blue" in Murray's New English Dictionary (1919) reports a first sighting of the term in the article on "silver" in Ephraim Chambers's Cyclopaedia of 1728.
However, many writers had used the term "sky blue" to name a colour before Chambers.
For example, we find "sky blue" in A Collection of Voyages and Travels (London: Awnsham and John Churchill, 1704), vol.
2, p. 322, where John Nieuhoff describes certain flowers: "they are of a lovely sky blue colour, and yellow in the middle".

The sense of this colour may have been first used in 1585 in a book by Nicolas de  Nicolay where he stated "the tulbant of the merchant must be skie coloured".

Displayed at right is the web colour sky blue.
Sky blue is a colour that resembles the colour of the unclouded sky at noon (azure) reflecting off a metallic surface. The entry for "sky-blue" in Murray's New English Dictionary (1919) reports a first sighting of the term in the article on "silver" in Ephraim Chambers's Cyclopaedia of 1728. However, many writers had used the term "sky blue" to name a colour before Chambers. For example, we find "sky blue" in A Collection of Voyages and Travels (London: Awnsham and John Churchill, 1704), vol. 2, p. 322, where John Nieuhoff describes certain flowers: "they are of a lovely sky blue colour, and yellow in the middle". 
The sense of this colour may have been first used in 1585 in a book by Nicolas de  Nicolay where he stated "the tulbant of the merchant must be skie coloured".
Displayed at right is the web colour sky blue.
Sky blue