To me one example of the kind of imagery you are referring to is when Marlow arrives at the Outer Station, and sees what he calls "the grove of death" and then meets the Accountant. Note his description of the grove of death:
They were dying slowly - it was very clear. They were not enemies, they were not criminals, they were nothing earthly now, - nothing but black shadows of disease and starvation, lying confusedly in the greenish gloom.
This description is juxtaposed with the appearance of the Accountant, who Marlow describes as follows:
When near the buildings I met a white man, in such an unexpected elegance of get-up that in the first moment I took him for a sort of vision. I saw a high starched collar, white cuffs, a light alpaca jacket, snowy trousers, a clear necktie, and varnished boots... He was amazing, and had a penholder behind his ear.
Notice how the use of "black" and "white" are contrasted. On the one hand, the description of the natives in the grove of death clearly dehumanises them, and juxtaposing this description with the Accountant could be argued to reinforce images of supremacy and "civilisation," yet at the same time Marlow is harshly critical of how the natives are victims of the colonial endeavour and we can infer his judgement of the Accountant for his ability to maintain what he thinks are vital and necessary measures in the face of such "barbarity."
You might want to re-read this excellent novella and look for ways in which white and black images are placed next to each other and analyse how they support or subvert colonialism.