The Nordic cross is the cleanest family in vexillology. A cross whose vertical arm sits closer to the flagpole than to the free edge marks a Nordic country, and there are only five to learn: Denmark is white on red, Sweden is yellow on blue, Finland is blue on white, Norway is a white-outlined blue cross on red, and Iceland is a white-outlined red cross on blue. Notice that Norway and Iceland are colour inversions of each other, which is the fastest way to stop confusing them. Denmark’s Dannebrog is generally described as the oldest national flag still in use, and the other four are all riffs on it.
The pan-African palette of red, gold and green traces back to Ethiopia, the one African state that kept its independence through the colonial era, so those colours became shorthand for African sovereignty. Ghana took them in 1957 and added a black star, and Senegal, Mali, Guinea, Cameroon, Togo and Benin all draw on the same set. A separate red-black-green tradition comes from the Pan-African flag of the diaspora. When you see a tricolour in these colours, your first guess should be West Africa, and the distinguishing feature is almost always a star, an emblem or the direction of the stripes rather than the palette itself.
The pan-Arab colours are red, white, black and green, taken from the flag of the Arab Revolt. Jordan, Palestine, Sudan, Kuwait, the United Arab Emirates, Syria, Iraq, Egypt and Yemen all use a subset. Most are horizontal bands with a red triangle at the hoist, or horizontal bands with a central emblem. Kuwait and the UAE both break the pattern in ways worth memorising: Kuwait uses a black trapezoid rather than a triangle, and the UAE runs a red vertical bar down the hoist instead.
The Slavic tricolours are white, blue and red in some order. Russia is white on top; Slovakia and Slovenia use the same three bands with a shield added; Serbia and Croatia reverse to red on top, with Croatia carrying its distinctive chequerboard. The palette alone will not save you here, so learn the emblems.
The Southern Cross group tells you you are in the southern hemisphere. Australia, New Zealand, Papua New Guinea, Brazil and Samoa all place the constellation on their flags. Australia and New Zealand both keep a Union Jack in the canton, and the reliable tell is the stars: Australia has six white ones, including a large Commonwealth Star below the canton, while New Zealand has exactly four, and they are red with white outlines. Six and white, or four and red. That single rule settles the most common mix-up in the southern hemisphere.
Finally, the Gran Colombia family: Colombia, Ecuador and Venezuela share yellow, blue and red bands inherited from the same nineteenth-century union. Colombia is the plain one, with a yellow band taking up half the flag. Ecuador adds a coat of arms with a condor. Venezuela adds an arc of white stars.