At various times throughout history colors as light as sky blue or as dark as dark navy blue have been used ( a selection apparently motivated by which color of blue dye was available at the time ) . When incorporated as part of the Union Flag , the navy blue color used was that of the Blue ensign belonging to the historic ` Blue Squadron ' of the English Royal Navy . 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 .