Similarities

Although the national flag is meant to be a unique symbol for a country, many pairs of countries have highly similar flags. Examples include the flags of Monaco and Indonesia, which differ only slightly in proportion; the flags of the Netherlands and Luxembourg, which differ in proportion as well as in the tint of blue used; and the flags of Romania and Chad, which differ only in the tint of blue.
The flags of Ireland and Côte d'Ivoire and the flags of Mali and Guinea are (aside from shade or ratio differences) vertically mirrored versions from each other. This means that the reverse of one flag matches the obverse of the other. Other than horizontal mirrored flags (like Poland and Indonesia) the direction in which these flags fly are crucial to identify them.
There are three colour combinations that are used on several flags in certain regions. Blue, white, and red is a common combination in Slavic countries such as the Czech Republic, Slovakia, Russia, Serbia, Slovenia, and Croatia as well as among Western nations including Australia, France, Iceland, Norway, New Zealand, the United Kingdom, the Netherlands and the United States of America. Many African nations use the Pan-African colours of red, yellow, and green, including Ghana, Cameroon, Mali and Senegal. Flags containing red, white, and black (a subset of the Pan-Arab colours) can be found particularly among the Arab nations such as Egypt, Iraq, Syria and Yemen.
While some similarities are coincidental, others are rooted in shared histories. For example, the flags of Colombia, of Ecuador, and of Venezuela all use variants of the flag of Gran Colombia, the country they composed upon their independence from Spain, created by the Venezuelan independence hero Francisco de Miranda; and the flags of Kuwait, of Jordan, and of Palestine are all highly similar variants of the flag of the Arab revolt of 1916–1918. The flags of Romania and Moldova are virtually the same, because of the common history and heritage. Moldova adopted the Romanian flag during the declaration of independence from the USSR in 1991 (and was used in various demonstrations and revolts by the population) and later the Moldovan coat of arms (which is part of the Romanian coat of arms) was placed in the centre of the flag. The Nordic countries all use the Nordic Cross design (Iceland, Denmark, Norway, Sweden, Finland, in addition to the autonomous regions of the Faroe Islands and Åland), a horizontal cross shifted to the left on a single-coloured background. The United States and United Kingdom both have red, white, and blue. This similarity is due to the fact that the first 13 states of the U.S. were former colonies of the United Kingdom. Also, Australia and New Zealand share a very similar flag, which stems from their joint British heritage. Both of these flags feature the Union Jack in one corner, both have royal blue background, and both have the Southern Cross as a prominent feature. The only differences between these flags is that the Australian flag has the Commonwealth Star below the canton, and that on the New Zealand flag, just four stars in the Southern Cross are presented, and they are five-pointed red stars with white borders. On the other hand, all five stars of the Southern Cross are presented on the Australian flag, and they are white with seven points, except for the additional smaller fifth star in the Southern Cross which has only five points on this flag. Some similarities to the United States flag with the red and white stripes are noted as well such as the flag of Malaysia and the flag of Liberia, the latter of which was an American resettlement colony.
Many other similarities may be found among current national flags, particularly if inversions of colour schemes are considered, e.g., compare the flag of Senegal to that of Cameroon and Indonesia to Poland. Also the Flag of Italy and the Flag of Hungary uses the same colours, in the same order, only the direction differs (the Italian flag is vertical and the Hungarian flag is horizontal)