Look at the olympic logo because every flag in the world has at least one of those colours.
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The first color that I can think of is Red and I am right!

According to worldflags101.com, we can see "red" in almost 75% of the national flags around the world. The second and third most poular would be white (almost 70%) and blue (about 50%).

Please check out:
http://www.worldflags101.com/faq.aspx

Thanks for asking this interesting question! I am going to ask my drinking buddies this question this weekend and see what kind of answers will I get.
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The vast majority of national flags use the traditional heraldic colors. Other than tradition, this was worked out in medieval times when distinguishing who was who from a distance was of great practical importance. The heraldic colors made their way onto flags when naval ships had to recognize each other from a distance. In national flags, only the boldest and easiest to read remained popular:

Yellow Or
White Argent
Blue Azure
Red Gules
Black Sable
Green Vert

Often families of flags have developed. New countries have copied older ones, either out of admiration for the country, or just following fashion. There is a family of flags popular among African nations that may be called the Pan-African colors. They are based upon two historical flags. The Ethiopian flag, the oldest independent state in Africa, was green, yellow, and red. The idealist Marcus Garvey designed a red, black, and green flag intended for a future unified black state. Thus the four colors green, yellow, red, and black became emblematic of black Africa and this flag family includes some 22 African countries.

The blue circle and seal in the middle of the Ethiopian flag that you have used as illustration was added in 1996. It is supposed to represent peace and unity, something Ethiopia has lacked ever since. The symbolism of flags tends to represent aspirations rather than reflections of reality.

Your Ugandan flag derives from the political party colors of the party that won the first elections. The crested crane, however, dates back to colonial times.
Source(s):
http://freepages.genealogy.rootsweb.ancestry.com/~jkmacmul/heraldry/colors....

The World Encyclopedia of Flags by Alfred Znamierowski

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