A seemingly simple question -- and one that is astonishingly difficult to answer (like all things concerning the dynamic Mississippi).
River color has been debated by artists and writers since time immemorial. It was probably the source of discussions by the first people who populated its shores, and probably woven into campfire songs and creation stories. A quick review of literature and paintings sheds illumination on this mysterious subject.
Its muddiness is of course most often associated with the most basic earth tone --brown. But which brown? TS Elliott found it to be a "strange brown god" in his Dry Salvages, and did not offer any further elaboration. William Faulkner saw darker brown tones in the river "rippling placidly towards the sea, brown and rich as chocolate between the levees who inner faces were wrinkled as though in frozen and aghast amazement... (Old Man).
As long as man has been painting he has been studying rivers and their various expressions. Visual artists look for nuances in the landscape and try to recreate it on their canvas. Here is a recent online conversation between painters about the color of the Mississippi River:
Painter 1: "I understand the river will change color as it moves down the USA, picking up various sediments, loosing the sediment as the river loses power and then getting muddier again as it gains power. Probably not an easy question to answer, but Warm Grey is a great start.... Our local river, the Fraser is a muddy river, but I seem to remember that it has a different muddy color than the Mississippi..."
Painter 2: "Americana has a craft paint called Mississippi Mud. It looks like dog that has been out in the sun too long..."
Painter 3: "It also gets a nice rainbow effect whenever an oil slick forms..."
Painter 4: "...needless to say it depends on weather, cloud cover, etc. I'd go for a 'mud' colored light to mid-brown and do washes of a greenish light grey followed by a fairly dark blue wash (the river at New Orleans is very deep)... Dog Poo brown is a good description. I remember long ago and far away, when I was a young'un, working out of Venice, LA, that was how the Captain of the first boat I worked on described it. Except he said something other than poo."
As physicists and painters both know, the color of the face of the river changes with the scenery, because the river surface is like a mirror, and will oftentimes reflect whatever is above or around it, taught in physics as the angle of reflection equals the angle of refraction.
Poets like Langston Hughes saw this quality of reflection from a train window crossing the St. Louis bridge when he wrote "I heard the singing of the Mississippi when Abe Lincoln/ went down to New Orleans, and I've seen its muddy/ bosom turn all golden in the sunset." Perhaps in response to her mentor in the above, Nikki Giovanni envisioned the bosom of the river turn "red in Memphis" in her rendition of the poem Tennessee (live reading, Jan 2012, Clarksdale, Miss.), perhaps as a reflection of the bluff city's bitter racial history.
Archeologists link river tones with local geology, "yellow chert is described as occurring in the upland areas of northern Mississippi and on modern Mississippi River gravel bars..." (Jane Rafferty: Time's River).
Watch for changes of water color as your journey progresses. Sometimes it is only a subtle change, sometimes dramatic. The Mississippi varies from muddy orange/yellow/green in high waters to a clearer orange/green in lower water levels. The colors of the various tributaries is of course partly result of the geology of the landscapes they drain. When the Ohio is flooding the color is darker browns & greens. When the Upper Mississippi is flooding you will see an even darker hue. When the Missouri is flooding there are more warmer muddy tones, more muddy-orange and muddy-browns, and also more fine suspended silicate. When the Arkansas is high it pours into the Mississippi with the muddy-yellows and oranges of southern plains. When it is low it is clear-green. Whenever you see greenish tinted river water it is probably richer in the blue-green algae which propagate in calmed waters.
Any confluence renders an interesting contrast which can be seen on the face of the water where the two rivers meet. With a little preparation paddlers can approach one of the many confluences on the Lower Miss and enjoy the constellation of patterns which erupt at these meeting places, spirals and boils and eddies of varying water colors, the colors often stay to the side of the river they came from and swirl alongside the other river for a long ways downstream until one color finally predominates. The collision of colors can be seen at the mouth of the Wolf, the St. Francis, the White, the Arkansas and the Big Black (and other rivers when they are flooding). For some reason the winning color is always the muddy color. And not just because of volume. This muddy truism comes from the original Big Muddy. If you hear someone call the Mississippi "the big muddy," this is only because its color derives from the Missouri. The true Big Muddy is the Missouri River. Its nickname is due to its rich silt laden waters which carry the sediments of the western and midwestern deserts, prairies and mountains.
The power of mud is first expressed above St. Louis where the normally smaller Missouri joins the Upper Mississippi and together they flow downstream through St. Louis to Cairo as the Middle Mississippi. Even though the Missouri is the smaller volume river, its muddy water colors overwhelm the darker & clearer tannin-rich colors of the bigger volume Upper Mississippi, and together they finally combine as a muddy brown river, in essence an expression of the colors of the Missouri. 180 miles further downstream the Ohio flows in from the East to join the muddy Mississippi. The Ohio normally carries twice the volume of the Mississippi. And yet its greenish waters become muddied by the Mississippi. It requires several dozen miles of side-by-side flow. The Greenish waters hug the Kentucky shoreline and the muddy brown waters hug the Missouri shoreline. But eventually they collide and revolve around each other and get stirred up into one homogenous mix. And which color predominates? Muddy brown of course!
The river is always telling a story, if only we could learn its language. With some careful artistic discernment you can tell if the Missouri is flooding -- all the way down in Mississippi by the water color, even though you are 500 miles away or further.