This isn't photoshop. It's a real river that changes color naturally.

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.