The flag of Missouri is a horizontal tricolor of of red, white and blue. These represent valor, purity, vigilance, and justice. The colors also reflect the state's historic status as part of FrenchLouisiana. In the center white stripe is the Seal of Missouri, circled by a blue band containing 24 stars, symbolizing Missouri's admission as the 24th U.S. state. The flag is described in Section 010.020 of Missouri's Revised Statutes.
Except for the Seal of Missouri in its middle, the flag is identical with the Flag of the Netherlands; this resemblance is not, however, intended to denote any special relation between Missouri and the Netherlands.
Update to design by Ken Morton. The flag retains the spirit of the current flag with some updates. Red and white stripes traditionally represent valor and purity, respectively. The blue represents three things: the permanency, vigilance, and justice of the state. The colors also reflect the state's historic status as part of the French Louisiana (New France). The center white wavy stripe symbolizes the Missouri River and the historic east to west direction of migrants moving westward through the historic Gateway to the West. The center badge is an abstraction of the state seal, with the star repressing the sate and the round shape representing the Gateway Arch and its reflection onto the Mississippi River. Posted by Preston Knapp 
MO Flag Proposal "apollohawkridge 1"
MO Flag Proposal "apollohawkridge 2"
MO Flag Proposal "apollohawkridge 3"
MO Flag Proposal "apollohawkridge 4"
Proposal for a flag for Missouri. All colors are taken from the current Missouri flag (from the seal). The shapes represent the St. Louis arch, and the colors are brown for the woods, green for the fields and light blue for the river. By Qaz Dec 2019 (details)
Design for a Missouri state flag. A revision of an earlier design by "Motx72". (Posted byKen Morton)
Missouri (Posted by Ken Morton)
Part of series of designs for state flags, all in blue and white. (Posted by Ken Morton)
This is my interpretation of the "banner of arms" of Missouri, as derived from the coat of arms, which I present here as an alternative to the state flag. To simplify the design, I reduced the number of colors to three. I did so by using the US coat of arms as found on the US Coast Guard Flag. I also replaced the bear, which lacks detail and looks like tired, with the bear found on the crest of Missouri Army National Guard coat of arms.
Missouri flag redesign proposal. As with other proposals, this design takes an element deep from within the coat of arms on the seal and elevates it to the whole of the flag. Mine includes a strip of yellow and a 1:1 aspect ratio.