The colours red, white and blue are derived from the coat of arms of the Ommelanden, and the white and green comes from the coat of arms of the city. The colours of the city are in the centre of the flag to represent the central location of the city.
The ratio of the flag is 2:3, the same as the Dutch flag The white cross has the width of 1/3 of the height of the flag and the green cross 1/9 of the flag height. The colors are defined as the Pantone-colors 032U (red), 300U (blue) en 355U (green)
In the year 1913 Van der Laars did a proposal to use a flag for the province of Groningen. His proposal was a flag that was based upon the flag of the Ommelanden. The blue stripe in the middle was exchanged for the color green of the flag from the city of Groningen. He also did two other proposals where he used the colors of the coat-of-arms of the province of Groningen.
The design of the current flag was made by Jan Tuin by then member of the Gedeputeerde Staten of the province of Groningen and later mayor of the city of Groningen. They couldn't agree upon with flag would become the flag of the province. Jan Tuin proposed to design a new flag, this design was accepted enthusiastically. On 17 February 1950 it was decided that this would become the flag of the province of Groningen. Six days later this rule was law.