hi! This question might be a job for /r/vexillology but to get you started, check out these related posts:

• Red ensign, Blue ensign, White ensign? - /u/QuickSpore and others give a rundown on when the different colours are flown

• Why were British colonial flags different colours? (I.E: Canada's was red while Australia's was blue) - /u/TheAlecDude discusses the adoption of Red or Blue Ensign by various countries

• Why did countries in the British Empire like Canada, British India, and the Union of South Africa have red British ensign flags as opposed to the blue ensign flags of Australia and New Zealand? - lots of chatter in this old thread, but scroll down to the nice post by /u/cuffx

The last two posts have been archived, so if you have questions for any of those users, ask here and mention their username to notify them