Thanks for the reply, it helped me solve my issue. It is not REST but that's fine. I found this graphql mutation to solve my specific case: https://shopify.dev/docs/api/admin-graphql/2023-04/mutations/metafieldsset. For future developers, that could ...

So I figured out how to set the metafield property with put request: {"product":{"id":632910392,"metafields":[{"key":"Colors","value":"newvalue","type":"single_line_text_field","namespace":"custom"}]}}But if the field has an existing value then I get...

Shopify doc on metafields:https://help.shopify.com/en/manual/custom-data/metafields?st_source=admin&st_campaign=metafields_footer&utm_source=admin&utm_campaign=metafields_footerAs seen in the image the user can add custom metafields to the products, ...