Overview
Croatian horizontal tricolour of red, white and blue inherited its colours, of course, from the Croatian coats of arms: Croatian chequered red and silver, Dalmatian blue with three golden lions, and Slavonian blue with red field with black marten bordered with two white wavy lines, and golden star above. All of the three coats were, in fact, used in different times for all the land, as it is with the name also. Later in 19th cent. the geographical meaning of these names was finally firmly established. The land was named in 19th cent Triunar (meaning one made of three) Kingdom of Croatia, Slavonia and Dalmatia. Therefore, it is not unusual that the flag was made of all three colours.
The first time that it was officially proclaimed was in 1848 and under the influence of revolutionary movements in Europe, a simple tricolour was made. The flag was, of course, without any coat of arms. However, it was used as local flag in a big empire, so it remained more or less internationally unknown, but it became one of the most important symbols of the Croats. Very often it can be seen on old postcards, most often together with Croatian coat of arms (normally not on the flag, but from time to time as a part of the flag).
After the end of the World War Croatia proclaimed independence, and a tricolour became the state flag, again without coat of arms. The collision with the Dutch flag didn't become, as a matter of fact, a problem, because the state was short living. After a short time, Croatia was united with Slovenia and Bosnia and Herzegovina in the State of Slovenes, Croats and Serbs (not to be mistaken with the later Kingdom of Serbs, Croats and Slovenes), and a month later united in a Kingdom under Serbian dynasty which already had annexed Montenegro. This state was then named Kingdom of Serbs, Croats and Slovenes, and took a neutral combination of pan-Slavic colours blue white red (both Serbia and Montenegro had red blue white and Slovenia white blue red).
The Croatian flag again remained as local flag, highly respected by the people, but half illegal, as the Belgrade government tried to unite all nations into a single Yugoslav nation. These efforts didn't succeed, and in 1938 a separate province Banovina of Croatia was made. It took, as one can expect, the red white blue tricolour and in official use the coat of the province in the middle. By this time many Croatian political parties used the Croatian tricolour with their respective symbol in the middle. The best known was the Croatian Peasant Party, HSS, with a chequered red white field bordered green.
After description of the Austrian-Hungarian flags used on sea during 19th century on eastern Adriatic, and after a paragraph on national colours (i.e. Croatian national tricolour based flags) used as house flags and owner's signals on ships beside the A-H ensign, Isaić writes: "For river navigation the flag hoisting was solved with legislation of 1869 according to which each ship was to hoist its national flag. The first Croatian river ship "Sloga" was owned by Savsko-kipsko d.d. from Sisak, acquired in 1844." This hints at least that the ships on rivers used national flags as opposed to the flag of the Dual Monarchy, and that this "national" is mean the flags of Croatia (Hungary, Austria etc. as appropriate). The year of 1869 is also the year of the Croatian-Hungarian Agreement with which the duties and authorities of the two kingdoms were in details defined.
Read more about this topic: Flag Of Croatia