"A White Heron," by Sarah Orne Jewett, is an excellent example of local color because of its realism. Local color refers to an author's use of the language (dialect) and manners of a particular group of people.
In this story, Jewett uses dialogue to show the difference between the "locals" and outsiders. Consider the differences between the following statements:
"Put me anywhere you like," he said. "I must be off early in the morning, before day; but I am very hungry, indeed."
"Dear sakes, yes," responded the hostess, whose long slumbering hospitality seemed to be easily awakened. "You might fare better if you went out to the main road a mile or so, but you're welcome to what we've got. I'll milk right off, and you make yourself at home.
The first statement is spoken by the ornithologist who is searching for the white heron. His language is more refined and he does not use contractions, in contrast to Sylvia's grandmother's slang ("dear sakes") and her contractions. He is not formally part of the same region (culture, lifestyle, manners) as Sylvia and her grandmother, which is why he knows his offer of ten dollars (so little to him) will be a great enticement to this poor family.
Jewett also adds local color by her use of conversational topics and practices common to the region in which Sylvia and her grandmother live.
Soon it would be berry-time, and Sylvia was a great help at picking. The cow was a good milker, though a plaguy thing to keep track of, the hostess gossiped frankly.
These two lines evoke the image of a farm-woman who is sitting down at the table for a quick gossip before beginning her homely chores once again; she spends her time thinking about picking berries during "berry-time" and fretting about the sporadic milk-giving habits of her cow.
Both the dialogue and the use of details in this story mark it as particular to a time and place, which is what the term "local color" means. Your question adds the idea of realism, but I wonder if you meant regionalism, as the same elements which signify local color are also indicators of regionalism. In any case, these details make the story believable and realistic, and this story is a good example of regionalism, as well.