Surprisingly, the first time the fog is mentioned, no color is given. However, as the narrative proceeds, we're specifically told that the fog is white. It also seems solid at that point. In fact, we're told: "When the sun rose there was a white fog, very warm and clammy, and more blinding than the night. It did not shift or drive; it was just there, standing all round you like something solid." That indicates that whiteness restrains action in this novel, making the fog a metaphor.
Rather interestingly, even though we are assured the fog is literal and there, it seems to heighten the passage of the riverboat into the unknown, and seems to draw a cloak between attacker and defender in the ambush. This could show the power of this "darkness" against the exploits of colonialism, and how that no matter how much a claim is made on land (through Leopold II of Belgium), the physical existance has no master or controller, but is at peace to waft at its own pace and will. This showcases the arrogance and hubris of the Pilgrims in their asusumed conquest of the region without contest.
The colour of the fog is white, "When the sun rose there was a white fog, very warm and clammy and more blinding than the night."