The bright yellow areas had very little diversity in 2000 but are experiencing the greatest rate of change. This pattern is evident from northern New England through vast swaths of the Midwest. It covers 56 million people. That’s one-sixth of the country that remained almost completely white all the way to 2000, but is now beginning the gradual change to a multicultural mix.

“When I was growing up in 1960s Dubuque, you didn’t see any other race, practically everybody was white,” said Janelle Lutgen, a Republican activist in Iowa. “I don’t think I worked with anybody of color, or went to school with anybody of color.”

But now Dubuque and the neighboring tri-State area of Iowa, Illinois and Wisconsin along the Mississippi River is bright yellow with change, enough to generate some friction.

[ If you’ve ever described people as ‘white working class,’ read this]

Dubuque had a Black Lives Matter march and rally earlier this month. The participants mirrored the Iowa community — they were virtually all white — but said that addressing division was a goal. Lutgen, the Republican activist, agreed that change doesn’t have to mean conflict.

“I think it’s more the unknown than the actual problem,” she said of neighbors uncomfortable with increased diversity. “It’s more a perceived problem than an actual problem.”

A different type of bright yellow spot is Hamilton County, Ind., a commuter haven just north of Indianapolis, where diversity is arriving in the form of rapid growth. This county’s population increased by 59 percent since 2000, and the community offers affordable housing, a thriving business community and excellent schools.

The arrivals are often professional and diverse. In 2000, the county was 93 percent white, while in 2014, it was 85 percent white, with the second-largest group being Asians, followed by Hispanics and African Americans.

Cities in this county made it on lists of “ best places to live” and “ happiest suburb.” Hamilton had the nation’s greatest increase in racial and ethnic diversity of any county with at least 50,000 people.