Both of these ships have Union Flag in the upper left corner of their banners, but one of the ensigns is red, while the other is blue. Why?
red is a civillian ship, blue is authorised by the Admirality
red is British, Blue is Australian
red is East India Company
red is a flagship
A British ensign in British maritime law may have with either a red, white or blue field, dependent on whether the vessel is civilian, naval, or in a special category. These are known as the red, white, and blue ensigns respectively.