According to Kevin Eastman one of the creators of TMNT and the African American acquaintance April O'neil's namesake comes from, she was going to be Asian. Eastman told that in Peter Laird's (the other creator) notes that April O'neil was Asian. However according to Peter Laird, he always in visioned her as possibly being of Irish/Scottish/English ancestry. If you ask me, it could have been true that Peter Laird wrote it and forgot about the original vision or maybe Kevin Eastman was mistaken.
There was a rumor that April O'neil was originally black - propagated around the time of Rise of The Teenage Mutant Ninja Turtles aired, where they race swapped April O'neil - and was whitewashed in the 1987 tv show. However comapared to previously established African American Baxter Stockman who was her boss, she was pale in comparison... in the literal sense. There still is the possibility of mixed-race until Kevin Eastman colored her in the comics as white skinned, before the deal with the cartoons. Most pictures that are used to prove she was African American was a picture of her in the original comic cover of Vol. 1 issue 11 (created/published in 1987 after which the comic had her colored white), which at the time perms were a popular fad in fashion and was based off of Eastman's girlfriend and name inspiration April Fisher, who did become Kevin Eastman's first wife.
static.wikia.nocookie.net/tmnt/images/7/7d/True_Stories.jpg/revision/latest?cb=20160524135408
Though do you know who was "whitewashed"? Previously established African American Baxter Stockman, who later appeared in the 1987 tv show as white. Though not April O'neil's boss as her backstory was changed. He later returned to being black in 2003 cartoon. And became white again in Mighty Morphing Power Ranger x Teenage Mutant Ninja Turtles crossover comic, which wasn't really based with the 1987's TMNT, but it was made aside the IDW comics... so maybe they did it so people would know it isn't the same canon? Who really knows.

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