In chapter 5 of Lord of the Flies, what is the significance of the "whiteness in the gloom" and the discussion that follows?

At face value, the phrase "the whiteness in the gloom" refers to the conch that Ralph grabs from Maurice's hands during the assembly Ralph calls late in the day after Jack's hunters had allowed the signal fire to go out. Because the light has faded, it is becoming harder to see the faces of the boys as they speak. Earlier in the chapter, describing the conch shell, Golding writes, "exposure to the air had bleached...

At face value, the phrase "the whiteness in the gloom" refers to the conch that Ralph grabs from Maurice's hands during the assembly Ralph calls late in the day after Jack's hunters had allowed the signal fire to go out. Because the light has faded, it is becoming harder to see the faces of the boys as they speak. Earlier in the chapter, describing the conch shell, Golding writes, "exposure to the air had bleached the yellow and pink to near-white." Now Ralph grabs at the white conch, taking it from Maurice, who has been defending the idea that a squid-type beast could come up out of the water. The boys have fallen into a chaotic argument, and Ralph blows the conch to restore order to the assembly. 

Next Simon, uncharacteristically, takes the conch to speak. He says, "Maybe there is a beast. ... What I mean is ... maybe it's only us." Simon, as the most insightful of the boys, has an understanding of "mankind's essential illness," but he has a hard time expressing it in words. He wants to explain that the thing the boys have to fear comes from within themselves, namely, from their depraved hearts. So he asks, "What's the dirtiest thing there is?" probably intending to explain that their own lack of morality is the "dirtiest thing." Jack ruins the discussion by swearing, probably using a word for excrement. This causes the boys to erupt in laughter, making any philosophical discussion about the boys' threat to themselves impossible. Even Ralph and Piggy have no understanding of what Simon was trying to say; Ralph later reprimands Simon for not saying there wasn't a beast.

In a sense, Simon stands out from the other boys as a light of truth, just as the white conch stands out in the gloom of the evening. Ralph blows the conch to call the boys to attention, and Simon speaks, trying to gain the boys' attention about the danger of the beast within. The "whiteness in the gloom" refers literally to the conch in the dusk, but figuratively points to Simon among the dark hearts of the other boys.