A diamond open access journal is an open access journal with no publication or submission fees. It is free for readers and authors, and is generally funded by universities, research centers or academic libraries.
Gold open access is publishing research in an open access journal, the articles are accessible immediately.
Green open access is generally uploading the accepted manuscript (last version of the article before publication, after peer review) to an open access repository (institutional or disciplinary) of an article that was not published open access. It is fully legal, but sometimes the publisher requires an embargo period, and adding a sentence like "this is the accepted manuscript of an article published by Wiley in xxx Journal on publication date, accessible at link with DOI" is usually necessary. The website Sherpa Romeo gives information on the conditions of each publisher: https://v2.sherpa.ac.uk/romeo/