Well, from a practical perspective, red and blue were easy to dye, and most flags today are based on derivations of old houses/cities/monarchs' flags.

For instance, the French flag is base don the flag of Paris (which is the french flag, without the White bar in the middle). The two colors are representative of two saints (Denis and.... I think Martin?) and, during the rebellion, the White was added in because it is the traditional color of France (think Fleurs du Lis).

The Dutch flag used to be blue, white and ORANGE-- the colors of the house of Orange, sensibly enough, but the orange dye kept reddening in the sun, and being sensibly Dutch, they eventually just adopted the red as the new third color on their flag (there's a lot more history to the netherlands and this is intentionally brief. I'm sure you could find someone to walk you through the entire evolution of the Netherlands and their standards, but it wont be me today).

I always thought the current Norwegian flag had a cool origin-- the designer didn't want it to be just red and white, since that would end up essentially being the flag of Denmark, and wanted to omit Yellow or a predominate blue as that evoked memories of their history with Sweden, so he went for Red, White and Blue because they were evocative of freedom, as seen in France, the UK, USA, etc.

The UK's flag is a combination of the flags from the different parts of the union, etc.

So basically-- at first it was easy, then it became tradition, then it became standards of freedom, now it's associated with that as a result.

Plus, they look good together.

Pretty much every country comes up with some symbolic explanation of their national colours ("blood" for red for example), but those are rarely the reason those flags were adopted in the first place. It's more common for a flag to be based on an earlier flag, at least in part. And it's not just red white and blue either.

So take the Dutch flag for example. It was originally orange-white-blue, but over the course of the 17th century it became exclusively red-white-blue, possibly because of how the early dyes would fade over time. The Dutch tricolour is thought to be the inspiration for the Russian white-blue-red tricolour. The Russian flag in turn inspired the Pan-Slavic colours, which can be found on the flags of the Czech Republic, Slovakia, Slovenia, Serbia, and Croatia. (Also Bulgaria, where the blue has become green.)

Another influential early flag was the British Union Jack, which was adopted in 1606 as a combination of the blue and white saltire of Scotland and the red and white cross of England. The flags of Fiji, Australia, New Zealand, Tuvalu are all based on British colonial ensigns, and Samoa's flag was probably inspired by that kind of design. The American flag is ultimately descended from the British flag (via the Continental Colours), and its flag inspired the flags of Liberia, Panama, Cuba, Chile, Malaysia, and the Philippines (plus Togo and Uruguay, which use other colour schemes.) T

You can trace these lineages back for the majority of the flags in the world. From the green-yellow-red Ethiopian flag we get the Pan African colours, used by nearly every country in Africa. From the red-black-green-white Arab Revolt flag we get the pan-Arab colours, used by nearly every country in the Arab World. The Danish flag gave us the Nordic Cross, used by Sweden, Norway, Iceland, and Finland. The Soviet flag inspired the flags of China, Vietnam, North Korea, and Angola. The French flag inspired the flags of Costa Rica, Haiti, Ireland, and Italy, and generally popularized the idea of the tricolour, which is used by dozens of countries. El Salvador, Guatemala, Honduras, and Nicaragua use variations on the old flag of Central America, which was based on the flag of Argentina. The flags of Colombia, Ecuador, and Venezuela have a common ancestor in the flag Gran Colombia. Cabo Verde, Kosovo, and Bosnia all somewhat based their flags on the flag of the European Union. Thailand chose its colour scheme because most of its allies in World War I were using it.

So basically the answer is that it's very common for national flags to be based on earlier national flags. Red white and blue is the most common colour scheme because it was adopted early on by some influential countries, but there are a whole lot of elements and colour schemes that get copied like that.

This was posted some time ago. I made a response there which gives a fair overview.

I'd also recommend crossing this to /r/Vexillology