This question has already been asked and answered on r/askhistorians before. The link to that post is:https://www.reddit.com/r/AskHistorians/comments/1bvgi7/why_are_red_white_and_blue_common_national_colors/

To summarise for those who tl:dr every flag in the world has their own origins not in an arbitrary choice of colour but in specific cultural significance. Basically every flag has its own story. Here are some examples from that post:

UK: Combination of the flags of England and Scotland and the Cross of St Patrick, Ireland's patron saint.

France: Based on the red and blue coronet worn by revolutionaries in the French Revolution with the white of the Bourbon (the Monarchy of France) flag.

U.S.A.: Ascribed a specific meaning to each colour, white is purity, red is bravery and blue is justice.

Netherlands: Originally orange, white and blue but they changed it due to red being more visible.

Haiti: Took the French Flag and ripped the white out to symbolise freedom from the white man.

Dominican Republic: Based on the Haitian flag that has the same colour scheme.