So... is Mitsuboshi Colors finished or not?
NO, it is not finished.
Why?
The anime finished before the manga:
Anime finished: March 25, 2018
Manga finished: June 27, 2020
MangaUpdates contains information about the anime adaptation and indicates that it is incomplete.
Watching Order (by release date)
Here you have all the anime relations we detected ordered by release date, so you can see the reason for this status in context and also use it as watching order.
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Synopsis
Mitsuboshi Colors centers around three young girls, Yui, Sacchan, and Kotoha, who call themselves the Colors and imagine themselves as the protectors of Ueno.
In reality, they mostly spend their days playing games, exploring their neighborhood, and involving the local people in their imaginative scenarios.
An old man named Pops helps fuel their adventures with props from his shop.
A local policeman, Saitou, often finds himself unintentionally caught up in their games, much to his initial annoyance, but he ultimately tolerates and even participates in their antics.
While their actions aren't serious crime-fighting, the Colors bring joy and a sense of fun to the everyday lives of the people in Ueno.