The fog that Marlow's steamboat gets caught in is white. But the white fog in this case is not the opposite of darkness; it simply compounds darkness. Fog not only obscures but distorts: it gives one just enough information to begin making decisions but no way to judge the accuracy of that information, which often ends up being wrong. Marlow's steamer is caught in the fog, meaning that he has no idea where he's going and no idea whether peril or open water lies ahead. He and the pilgrims he is carrying are frightened they will be attacked in the fog. One man states,‘We will all be butchered in this fog". Thus the confusion and darkness of the river is intensified by the fog.
The fog that engulfs Marlows' steamboat on the river is white. The fog distorts Marlows vision. White in general is usually pictured as "good" or "pure" but in Heart of Darkness light obscures the truth and makes it difficult to see true intentions. White is an outter cover, a banket that humans created to hide the dark black truth about the savage nature of man. For example the companies headquarters in Brussels is referred to as a "whited seplecure" The whited seplechure is referenced in Mathews biblical book and in it he describes a whited seplecure as something that is white and beautiful on the outside but contains horrors within.