Quote 70
"I know that the sunlight can be made to lie too, yet one felt that no manipulation of light and pose could have conveyed the delicate shade of truthfulness upon those features." (3.48)
Marlow doesn't trust the sunlight anymore because he has learned from his experience in the interior that light can be deceitful or hellish. (Plus, it can give you wicked sunburns.) However, he trusts the Intended because he believes women are naïve. Hm. We're thinking that's a bad idea.
Quote 71
"I was fascinated. It was as though a veil had been rent. I saw on that ivory face the expression of sombre pride, of ruthless power, of craven terror - of an intense and hopeless despair. Did he live his life again in every detail of desire, temptation, and surrender during that supreme moment of complete knowledge?" (3.42)
As Kurtz dies, Marlow sees a parade of negative emotions pass over his face—pride, ruthlessness, terror, and despair. Contrast that with the white, calm connotations of "ivory," and you'll see why this book has us scratching our heads so thoughtfully.
Quote 72
"His [Kurtz's] was an impenetrable darkness. I looked at him as you peer down at a man who is lying at the bottom of a precipice where the sun never shines." (3.40)
Kurtz is so evil now that light can't even touch him. Metaphorically.