The flag currently used by Lithuania is a horizontal tri color of yellow, green and red.
It was formally adopted on 20 March 1989, two years before Lithuania regained independence and after the Soviet Union dissolved.
There was a brief Nazi occupation from 1941 to 1945. The USSR regained control when WWII ended in 1945.
The flag used before the collapse of the Soviet Union was a generic Soviet red flag with the name of the country on it and narrow green and white stripes at the bottom.
At this time many other nations were creating new flags that resembled the French tri color which represented freedom.
The red, green and yellow flag of Lithuania was inspired by native Lithuanians living in other countries and was first flown when independence from Russia was granted in 1918.
Prior to this flag was a White Knight on a horse which had been used for centuries. When the USSR took control in 1944 it outlawed the flag. In 1989 Lithuania regained independence from the USSR.
It is a criminal offense not to fly the flag on Independence Day and you can be fined. In 1992 the government defined the exact colors and design after they were adopted in 1992.