What the colors are doing
A national flag has very little room to work with, so the colors have to carry most of the argument. Three countries make the point well, because all three use ordinary colors to say quite different things: Algeria, Argentina and Botswana.
Algeria: green and white
Algeria's green stands for Islam and for fertility, which in a country with deep Islamic roots is less a design choice than a statement of fact.
The white is for peace and purity, which reads differently in a nation that spent years under colonial rule before winning independence. A blank half of a flag is a reasonable thing to want after that.
Argentina: sky blue and white
Argentina's sky blue and white are usually explained as the sky and the country's waters, with white for peace and unity. The pairing is also bound up with the fight for independence, so the calm reading and the martial one sit on the same piece of cloth.
Botswana: light blue and black
Botswana's flag is light blue and black. Blue is water, which in Botswana is not a decorative choice, and it also carries the usual association with peace. Black stands for harmony among the country's different populations.
The same colors, different arguments
Put the three side by side and the overlaps are superficial. Green and blue both point to natural elements, earth and water, but Algeria uses its colors to talk about religion, Argentina about landscape and Botswana about who lives there. The palette is shared. What each country means by it is not.
Why color works at all
Colors get an emotional response before anyone has parsed the symbolism, which is exactly why flags rely on them. Seeing your own flag at an international event lands somewhere between exciting and reassuring, and no amount of explanation about what green stands for produces that effect on its own.
Flags do change, and globalization pushes meanings across borders in ways that were not possible before. The core job stays the same: a flag has to identify a country and hold its people together, and it has to do it in two or three colors.
