The Flag of Scotland is a white saltire, a crux decussate (X-shaped cross) representing the cross of the Christian martyr Saint Andrew, the patron saint of Scotland, on a blue field. It is named the Saltire or the Saint Andrew's Cross. In heraldic language, it may be blazoned Azure, a saltire argent.
At various times throughout history colours as light as sky blue or as dark as navy blue have been used (a selection apparently motivated by which colour of blue dye was available at the time). When incorporated as part of the Union Flag, the navy blue colour used was that of the Blue Ensign belonging to the historic 'Blue Squadron' of the English Royal Navy.
Although this navy blue colour was used specifically for depicting the Union Flag on maritime flags on the basis of durability, it soon became standard on Union Flags, both on land and at sea. This navy blue colour trend was adopted for the Saltire itself by many flag manufacturers, resulting in a variety of shades of blue being depicted on the flag of Scotland ranging from "sky blue" to "royal blue" to "navy blue". Eventually, this situation resulted in calls to standardise the colour of Scotland's national flag.