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Novels are not a visual medium. Your readers never actually see your characters. So unless the racial background of your characters is relevant for your story, you can easily get away with never ...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/41596 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
Novels are not a visual medium. Your readers never actually see your characters. So unless the racial background of your characters is relevant for your story, you can easily get away with never actually telling your reader their skin color or eye shape. You said that you had "forgotten about their races because it wasn't important to you". When it isn't important to you, then it likely isn't important to your story and it isn't important to your readers either. So just never tell the reader what race your characters are and let them insert whatever they feel their race should be. In the unlikely case that anyone then gives you feedback asking "why is there no character of [ethnicity] in your book?" ask them back "why are you so sure that [random character] isn't [ethnicity]?". When they give you a good reason why they think that's implausible, then there are two possible reasons: 1. The reader has racist preconceptions and applies them to your characters (like "someone of [ethnicity] would never have [job] or do [action] or hold [opinion]"). Some people just are that close-minded. There is nothing you can do about that. 2. The reader is actually right and you did write the character as a racial stereotype. In that case, make sure your characters are not primarily defined by their race.