Rivers in the Amazon differ greatly in color according to the type and quantity of silt. Rivers that arise in the Andes are rich with silt and consequently have a pale brown color, like coffee with cream. These are called "white water" rivers (nothing to do with river rafting!). The largest such river is the Amazon main stream itself. Rivers that arise in ancient cratons (rocky massifs) such as the Guyana shield in northeastern Brazil, have low silt levels and most of the suspended solids derive from plant matter. This material gives the water a dark appearance, like coffee without cream. These are the so-called "black water" rivers. The biggest of these is the Rio Negro, whose name itself means Black River. Where these two kinds of rivers meet, the waters do not blend right away but remain separate for many miles downstream. The most spectacular of the "meeting of waters" is where the Rio Negro runs into the Amazon main stream, a phenomenon visible from space. (See Scenery meeting of waters for what this looks like on the river itself.
This MISR image (text added) shows a region about 70,000 square miles centered on the Brazilian city of Manaus, largest city in lowland Amazonia. The colors are exaggerated, but clearly show the how the black water Rio Negro converges with the "white water" Amazon main stream. At this point is the "Meeting of Waters" a must-see for visitors to the region. The effects of road building are clearly seen.
This image taken from the space shuttle, is of much the same region as in the above picture, but taken at a time when the river was in high flood. Compare how much of the land is flooded by water. The colors represented show the scene much as it appears from space.