Yes it's right.
In fact none of the people before the 60s of the 20th century did not have a flag.
In the Middle Ages, in particular, are not there nothing to the national flag.
Then there are the armorial banners rulers and nobles and military standards of military units.
Only with the emergence of the nation begins to emerge of a national flag, and that's the end of the 18th and beginning of the 19th century.
Before that there is no national flag.
The first national flag evolved from naval flag that began to be used on land, and to have a nice example of the Holland (red-white-blue) or Russian (white-red-blue) flag.
Even the flag of the rebels I Serbian Uprising (red-blue-red) was a naval zastva Ottoman Empire in Orthodox (for Muslims was red-green-red, red for the Jews-yellow-red and katrolike red-black-red) .
In the early 19th century, under the influence of poanslovenskog movement, the Serbs started to use red, white and blue flag with the Croats.
After a year the Turkish sultan a firman awarded Serbs red-blue-white flag that bears a striking resemblance to a modification of the Turkish naval flag for Orthodox (red-blue-red).
Parallel Montenegrins use the most of the 14th century armorial banner Crnojevića which is on the red double-headed eagle in gold or silver.
That banner was used during the vladikata, during a secular state Petrovic, until the early 20th century, when it introduces a tricolor flag in the arrangement of colors of the Serbian flag.
The Montenegrins were not acknowledged any multicolored flags.
When the Russians attempted to foist their Russian white-blue-red flag, during the war with Napoleon in the Boki, they are cast with a sneer.
Neither the Serbian flag did not want to use considering it a symbol of recognition of the Turkish authorities, since it is a Turkish sultan granted the Serbs.

Pobedniče do not understand some things.
Montenegro as a political entity disappears from the face of the earth (and Montenegrins as well as political, ethnic and national categories) and annexation became part of the Kingdom of Serbia.
Zeta Banovina was an administrative unit within Serbia (because the Savska and Primorska Banovina became Croatian Banovina, and Slovenia was the Drava Banovina, the rest of Serbia).
Montenegro did not exist from 1918 to 1941 under domination Karadjordjevica and Serbian national elites.

you are confirming all of my previous posts actually. longest use a real state flag was our tricolor. It was flag of Montenegro, not flag of Serbia. even though the colors were in same order it represented our country.
how doid we get it, or how did any state get its own flag is not of real importance. Krstaš barjak for example was modified Turkish banner.
Regarding the status of Montenegro in Kingdom of Yugoslavia, it was the same status that every other former state had, including Serbia.