The Saltire in heraldry is described as a white cross on an azure background. It dates as a flag in Scotland from 832 and a battle near Athelstaneford in the East Lothian between Picts, Scots and Angles. Azure covers a range of some 300 colours, all of which would be valid.

Until 2003 there was no official blue in the saltire so multiple different ones have been used (the Royal Navy for instance now uses Pantone 280C in the Union Jack but has used other darker colours in the past as they last longer at sea). See publication BR20 for the Union Jack and colour codes

In February 2003 the Scottish Parliament's Education, Culture and Sport Committee acting on a petition recommended that Pantone 300 be used. This followed input from the Lord Lyon amongst others on the appropriate colour. Pantone 300 was chosen as being closest to the blue of the sky and in keeping with the origin myth.

Out of interest do all the countries flags with a union jack in the corner use p280