Totally depends on what data you are displaying and what type of visualizations you are using. There are a number of themes available in Looker Studio that you choose from, or you can create your own. You can automatically set a palette based on an image, like a school logo, by bringing it into your report, right clicking, and choosing Extract Theme from Image.
You can learn more about themes here https://support.google.com/looker-studio/answer/9209811?hl=en&sjid=7899071593274641989-NA
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