Referencing Du Bois' contention, the "color line" is such a volatile topic because it impacts so many in so many different ways. The challenging element in assessing the role of race and ethnicity in consciousness is that different experiences fill the landscape. It is hard to assess what "the color line" exactly means because for so many it means different realities. Some of these are psychological experiences, others are professional, and some are economic experiences. The color line's impact on the lives of individuals is diverse. Full exploration of this diversity of narrative impact is one of the reasons why it is such a volatile topic.
For Du Bois, another reason why the "color line" remains a volatile topic is because it creates a "double consciousness." People of color who live on one side of the color line are forced to look at their being in the world as representative of both what it is like to live under the struggle of the color line and what "the other" is like. For those who experience the brutality of the color line, this double consciousness is something that plagues and continually gnaws at the individual. This persistent condition is one fo the reasons why Du Bois sees it as a volatile and challenging topic which has been a part of American culture and what he would say as remaining as a part of American culture even today.
I think that this is the case because both blacks and whites have relatively good reasons to feel like they are being victimized. This makes both sides angry and helps to make the issue more volatile.
Blacks have good reason to be angry because they have been systematically mistreated up to, at most, 45 years ago. The effects of this discrimination and mistreatment cannot possibly be cured in such a short time and so blacks still suffer both from racism in the present day and from the effects of past racism. Therefore, it is logical for them to feel victimized.
At the same time, whites can make a case for being victimized for no good reason today. Even though blacks have been discriminated against in the past, it is very easy for a white person today to argue that they themselves have done nothing wrong and should not be punished for the sins of past generations. Giving a black person preference over a white person today to make up for past wrongs seems wrong to many people. Thus, whites can legitimately feel victimized as well.
Since there is no clear right way to deal with the problem and since both sides seem in some ways to be in the right, the issue remains volatile.