If colors are referenced in their most basic form, the seven most common national flag colors (in no particular order) are red, blue, yellow, green, white, black and maroon. If colors are referenced in shades or variations of light and dark, however, then the answer changes to (again, in no particular order) red, dark blue, light blue, yellow, green, white and black.
Virtually all of the flags of the nations of the world contain red, blue, green, yellow or black. This is why the rings on the Olympic flag are those colors. Red is the most common color represented on national flags around the world. According to World Flags 101, 75 percent of all national flags contain the color red. The second most common color is white, which is found on about 70 percent of all national flags, followed by blue, which is found on roughly half of all national flags around the world. Like designs, colors on national flags often have meaning when used individually as well as in combination. Red, white and blue in combination represent freedom and revolution, while individually they convey courage, peace and freedom, which are common messages that nations like to send about themselves.