This chart (public domain from Wikimedia Commons) illustrates the question. Is there an explanation for why the Union Jack have a darker shade of blue than the original Scottish flag that it's based on?
In 1606, people didn't have pantone guidelines to keep colours consistent - nor did it matter. Heraldry only has a limited number of colors. Variations on blue exist but are not standard, so any blue could be used.
Various shades of blue have been used in the saltire over the years. The ground of the current Union Flag is a deep "navy" blue (Pantone 280), which can be traced to the colour used for the Blue Ensign of the Royal Navy's historic "Blue Squadron". (Dark shades of colour were used on maritime flags on the basis of durability.)
Adapting the colours of the naval ensign made sense because the Union Jack was originally meant for naval use only. See the original decree:
Whereas, some differences hath arisen between Our subjects of South and North Britaine travelling by Seas, about the bearing of their Flagges...