We recognize it, too, in popular culture—though there have been more gains in recent years, given that movies with black casts have become "surprise" box-office hits.

I think it best to limit our consideration to the United States because this is the country that DuBois was describing, and it is a nation that has continued to privilege its white citizens while subordinating its non-white citizens, particularly black people. White supremacy, or some semblance of it, exists in other countries, especially other countries in the New World. Brazil, which has the second-largest population of black people in the world behind Nigeria, is a good example. However, the United States is unique for both having fought a civil war over slavery and for maintaining a system of apartheid, or segregation, to control the descendants of its slave population.

In regard to the previous educator's examples, the Civil Rights Act made racial discrimination in public accommodations illegal, but it did not and could not eliminate the problem of discrimination altogether. The most glaring contemporary example of discrimination is the way in which police power is meted out toward black people as opposed to whites. The police are used to circumscribe limits on where black people are allowed to be, based on lingering stereotypes about black people being violent, likely to commit theft, and altogether socially undesirable.

The self-segregation that results in social circles (though, studies have shown that whites are more likely than any other group to have no friends outside of their race) reflects the racism that permeates every facet of society. Groups composed of people of color usually sit together to protect themselves from the kind of rejection that DuBois described when a white girl in his class rejected his holiday card. He suddenly became aware of himself in a way that made him feel bad about who he was. The color line does not only separate, but it is drawn by those in power to reinforce myths of inferiority and superiority in the interest of the dominant group maintaining power. In this regard, the color line remains as visible during DuBois's time as it is during ours, which is why we still read his work.

Good question!  In Du Bois's quote, he specifically mentions the color line being the problem of the 20th century and references its existence in all parts of the world. The term itself refers to the division between races--a figurative line over which one race cannot cross.

I don't think that anyone could logically argue that color lines are now nonexistent, but in some places of the world, the lines are "fainter"--they don't have as much power as they one did. In the United States, for example, we undeniably still struggle with race relation issues, but the Civil Rights movement pushed through significant positive changes.  While the movement and act did not obliterate the color line, they certainly made it less restrictive.  Black Americans were able to cross the line into jobs that were previously "off-limits," and the desegregation of schools began.

That being said, we still see a great deal of self-segregation not just in America but also around the globe. Some Asian countries significantly discourage intermarriage, and my students still segregate themselves during lunch and for other activities.  While neither example represents a mandatory color line, each at least demonstrates that for some reason we humans resort to "drawing" the lines. 

DuBois was interested in progress and progress has certainly been made since the publication of The Souls of Black Folk in 1903. Voting rights and equality of citizenship have been more fully and indiscriminately extended to ethnic groups of all stripes and opportunities for quality education, health care and employment have all risen for minority groups in the United States.

The notion of "double consciousness" that DuBois articulated seems to be less distressing today than in was a few decades ago as popular culture moves into modes of increasing diversity and better representations of the American diaspora, so to speak. (Although, the #Oscarsowhite Academy Awards issues of 2016 suggest that there is still progress to be made in this area too.) The idea of "being Black in public" is not as fraught or freighted as it was during the 1930s to the 1960s, as depicted in works like The Help and To Kill a Mockingbird. 

There has been progress, yet we continue to see discrepancies between ethnic groups that echo Dubois’ proclamation that the “duty and the deed” set before contemporary America is “the problem of the color line.”

U.S. Census Bureau data shows that Blacks, Native Americans and Hispanic or Latino Americans each show poverty rates at above 20%. Each of these groups shows a rate of poverty that is roughly double that of White Americans.

As this post is being written, an presidential election is underway and voting rights have come again into question as certain states have asserted state’s rights to govern their own voter restrictions in ways that are seen by some as, in effect, disenfranchising voters that will be disproportionately associated with particular (minority) ethnicities. This creates a scenario wherein American society is again debating the amount of progress that has been made and may still need to be made in regards to equality of rights (and access to government, civic institutions, etc.).

In light of numerous headlines in the last decade depicting law enforcement violence against Black males and females, conversations have also turned to question of equal protection under the law.

With a question being posed here as to the relevance of the “color line” notion as a problematic factor in social, economic and political American life, these examples seem to strongly suggest that this notion remains relevant.

The critic DuBois presents in The Souls of Black Folks is importantly subordinated to an articulation of goals and aspirations. He paints a picture of successful American life where the color line has been erased. It has not yet been erased, and so his work remains apropos of the contemporary moment. But the ideals expressed in his work are therefore also still relevant and compelling goals.

“Work, culture, liberty -- these we need, not singly but together, not successively but together, each growing and aiding each, and all striving toward that vaster ideal […] the ideal of human brotherhood” (7).

While the reality of the notion in question here has certainly shifted, we might say as a final work that the idea of ethnic difference and problems of race-oriented inequalities has not disappeared.