It is a headline that reads as if it were ripped from a Saturday Night Live skit: Julia Roberts was once the favourite to play the legendary African American 19th-century abolitionist Harriet Tubman. The revelation came from Gregory Allen Howard, the screenwriter and producer of the recently released film Harriet, in a Q&A earlier this month.

“I was told how one studio head said in a meeting: ‘This script is fantastic. Let’s get Julia Roberts to play Harriet Tubman,’” Howard said. “When someone pointed out that Roberts couldn’t be Harriet, the executive responded: “It was so long ago. No one is going to know the difference.”

The suggestion, it must be noted, was made in a Hollywood landscape that predates the wrath of Twitter threads – Howard wrote the script for Harriet in 1994, but only managed to get the project off the ground last year. The practice of whitewashing, however, has in one way or another been rife in the film industry for decades and has simply adapted over time. Outright black/brown/yellowface in order to keep minority actors off our screens – Mickey Rooney’s performance as Mr Yunioshi in the 1961 film Breakfast at Tiffany’s; the droves of white actors portraying Puerto Ricans in West Side Story; or, as recently as 2007, Angelina Jolie as the French-born, Afro-Chinese-Cuban-Dutch Mariane Pearl in A Mighty Heart, complete with darkened skin and corkscrew curls – is now considered politically incorrect. But rewriting the race of a character as white in order that a white actor can play them is still considered normal (take the casting of Emma Stone as a woman of Asian and Hawaiian heritage in Aloha; William Mapother in World Trade Center playing a character who in real life; was black, or the Harry Potter producer’s transfiguration spell that saw Lavender Brown, a non-speaking black character, magically become white after her role became central).

Although increasingly challenged, whitewashing has not been banished entirely. Roberts as Tubman couldn’t happen today, but producers ignore these scruples when dealing with fictional characters: in 2010, the white actor Jake Gyllenhaal was cast as the Prince of Persia; in 2015, Rooney Mara played the Native American princess Tiger Lily; and, the year before, Joel Edgerton played Rameses II and Christian Bale played Moses in Exodus: Gods and Kings.

It is particularly galling because Hollywood is hardly brimming with parts for actors of colour. At least Ridley Scott was honest about Hollywood’s positioning on the issue. “I can’t mount a film of this budget … and say that my lead actor is Mohammad so-and-so from such-and-such,” he said. “I’m just not going to get it financed.”

Hollywood has changed since 1994. Yet I have no doubt that the rooms in which these conversations about castings and financing are taking place are still as whitewashed as ever.

• Yomi Adegoke is a Guardian columnist