I want to say late sixties? Jane Yolen wrote about it in Touch Magic.

To quote:
"Another author forced by changing attitudes to rework published material was Roald Dahl. In Charlie and the Chocolate Factory, the original Oompa-Loompahs were little chocolate-colored pygmies who had been discovered in darkest Africa, and who now play their banjos by the side of the chocolate river in Willa Wonka's factory. There they are used by Wonka in a variety of experiments. But - my! are they happy.

"I was a junior editor at Knopf in the early '60s, working on the manuscript of Charlie and the Chocolate Factory, having held my job there for less than a month. Dahl was asked by the editor-in-chief to make the Oompa-Loompahs some color other than black, and to drop the references to playing the banjos by the side of the river. But Dahl replied, "Racism is an American problem," and insisted the manuscript not be changed.

"Years later, when racism was also a British problem, Dahl bowed to pressure and changed the Oompa-Loompah's to tiny green-skinned people, shifting their place of origin to a South Sea island. This only slightly mitigated the problem of a different-skinned people being held in semi-benign captivity for the reward of food and a place to live."