Why do so many national flags contain the color red?

A very non historical answer. For what it's worth. I'm answering the question "why do people like or notice red colour more?" and extrapolate to flags. Many of the assertions below are well recognised in evolutionary biology but I did not look for sources for each of them.

Most land plants are green because they use chloroplasts to power the synthesis of organic molecules from carbon dioxide (Calvin cycle). For land plants the applicable lineage of chloroplasts are green (red for some sea weeds chloroplasts).

Red is the complementary colour to green (as all artists well know). Which means that red and green offer the strongest contrast. Another pair of complementary colours is blue and yellow (=> Swedish flag anyone? :).

There is therefore an evolutionary pressure for plants to advertise ripen fruit as red and conversely hide immature fruit as still green. Fruit consumers (who pay for their consumption by disseminating the seeds) have synchronously progressively experienced the complementary evolutionary pressure to see from far away and to be attracted by red objects. For instance the animal species most receptive to red are African primates - while most other animal species don't see red at all. Notable exceptions are birds (heavy fruit and berry consumers) and crustaceans (not sure why because red is rare in the sea). Similarly aphids are attracted by light green (young poison free leaves) and pollinator insects by ultraviolet (to spot flowers).

So young primates like colours (ever noticed marketing techniques to have your kid grab their yoghurt pack in the shops?) because in their original rain forest habitat it's a competitive advantage to be attracted by vitamin and nutrient loaded fruit. We, as human adults (with interestingly enough regional differences) are still very fond of red.

It remains to be seen of course whether bees national flags predominantly contain ultraviolet motifs and aphids flags green ones. :)

Overall flags, designed as group emblems, must express pride and possibly inspire awe (red is also used to advertise power in some primates - Gelada baboons chest for instance).

About regional differences - this is my own interpretation here. Northern hemisphere (colder countries) flag have also a lot of blue because nice weather (blue sky) allows to synthesize more vitamin D and it's a competitive advantage for its inhabitants (originally coming from the African plains) to have blue eyes, light skin (with less melanin) and to research exposure to sun light.

Conversely African flags were light is plentiful have very little blue but are very colourful.

Red dyes were some of the earliest developed, and thus more likely to be used for banners and flags. see http://www.straw.com/sig/dyehist.html

Probably a lot of flags contain red because red represents courage, revolution, hardiness, blood, and/or valor.

Red color also depicts sacrifice the hardships faced for a nation to be made. I believe there is a scientific reason too for having red among the colors in national flag. That is red being the color with the greatest wavelength in our visible spectrum red can be viewed from great distance and in case of the national flag which identifies a nation the color red makes sure that the flag is noticed.

Flags were used as a standard in battle. Red is an arresting colour for mammals and birds, which is why most ripe fruit is red. You can imagine that red is likely to be more visible and less likely to blend in than blue or white (sky) grey or black (smoke) or green or brown (trees, landscape etc.)

As was pointed out by others, colours in national flags are derived from older coat of arms or other symbols derived from medieval heraldry. That one itself become more or less formalized in the early 13th century, from a habit taken (especially during the Crusades) to hoist highly distinguishable colour patterns so as to promote warrior recognition on a dusty battlefield. Thus a relatively small set of primary colours, including red, which is very visible in a countryside landscape (the human eye is most sensitive to green, not red, but Nature is full of green so a green flag does not stand out the way a red flag does).

Among heraldry "rules" is the one about tinctures that says that you should put only a metal on a colour, or vice-versa, but not metal on metal or colour on colour. "Metal" is or (yellow) or argent (white); other tinctures are "colours". This rule is one of maximum visibility: to make details visible, they must be painted with tinctures that don't blend. These rules can still be seen in action on car license plates; for instance, old French plates were white-on-black, while recent ones are black-on-white or black-on-yellow; Belgian licence plates are red-on-white; and so on. A consequence is that most coat of arms contained some white or yellow (hard to avoid, with the rule explained above), which now shows on national flags.

It is noteworthy that flag evolution has not been straight from medieval banners to national flags; maritime flags played an important part, and visibility (in a nautical context) was the whole point of such flags.

While the above talks about plausible reasons why red would be most favoured (mainly because it is highly visible), this may be all purely coincidental. Worldwide statistics on national flags, by definition, work over nations, and a lot of nations were created in recent history. In particular, most of Africa (now 54 UN members) became independent countries in the last 70 years, and "invented" their flags at that time. Most of them reused some or all of the colours from the Pan-African flag, including the red. Warfare technology being what it is now, flag visibility can be said to no longer have any practical consequence; symbolism is a much stronger force in national flag colour selection. These African flags alone account for more than a quarter of current national flags, so such coincidental effects cannot be dismissed easily.

A similar effect may be observed in formerly communist states, who tended to use the "communist red" in their flags (e.g. China, Vietnam).

Yet another case is the Red Ensign that "pollinated" a lot of ulterior national flags thanks to the ubiquitousness of the British Empire in the late 1800s. For instance, Canada's flag red part is a deliberate reference to England (while the white part is explicitly a reference to the French pre-Revolution inheritance).

The colors come from the heraldic tradition, especially European and former European colonies' flags and coats of arms. In this summary on heraldry the basic heraldic colors are:

• Yellow (Gold)
• White (Silver)
• Red
• Blue
• Green
• Purple
• Black

If you check existing flags, you will find that they are almost all made in these colors. According to the link provided by Lohoris, the sum of the area of heraldic colors on flags is more than 99.6% of the area on flags worldwide. Many flags come from the medieval era, when heraldry was taken seriously. Later (even nowadays) these same colors have served as a guideline to make flags. Red is not really overrepresented in flags (red area: 30.3%, second is blue 21.15%). A weak preference comes from "Color psychology". There are studies on color's effect on the human mind. It is an another topic in Psychology, worth mentioning, but this part is not really just History. Red can represent many abstract thoughts from love, blood, courage and many others.