Wikipedia says:
"Throughout the history of fabric production
natural dyes have been used to apply a form of colour,
[14] with dyes from plants, including
indigo from
Woad, having dozens of compounds whose proportions may vary according to soil type and climate; therefore giving rise to variations in shade.
[15] In the case of the Saltire, variations in shades of blue have resulted in the background of the flag ranging from
sky blue to
navy blue. When incorporated as part of the
Union Flag during the 17th century, the dark blue applied to Union Flags destined for maritime use was possibly selected on the basis of the durability of darker dyes,
[16] with this dark blue shade eventually becoming standard on Union Flags both at sea and on land. Some flag manufacturers selected the same navy blue colour trend of the Union Flag for the Saltire itself, leading to a variety of shades of blue being depicted on the flag of Scotland.
[17]
These variations in shade eventually led to calls to standardise the colour of Scotland's national flag,
[18] and in 2003 a
committee of the
Scottish Parliament met to examine a petition that the
Scottish Executive adopt the
Pantone 300 colour as a standard. (Note that this blue is of a lighter shade than the Pantone 280 of the Union Flag). Having taken advice from a number of sources, including the office of the
Lord Lyon King of Arms, the committee recommended that the optimum shade of blue for the Saltire be Pantone 300.
[19] Recent versions of the Saltire have therefore largely converged on this official recommendation. (Pantone 300 is #006CB4 as hexadecimal
web colours.)