I think this is best explained by the images below from the Australasian Open Access Strategy Group.
Traditional publishing works like this:
Researchers create their outputs, and then they transfer copyright to publishers with no rights retained. The outputs are then locked behind paywalls, and even when users have access to them, they generally cannot share them with others outside their educational institutions.
Green Open Access works to break down that process.
With Green Open Access models, researchers create their outputs, and then they transfer most of their copyright to publishers, retaining the right to put a manuscript copy in an open access repository. Users still must pay to see the official published version, but the manuscript can be accessed by everyone for free.
Gold Open Access is slightly different.
With Gold Open Access models, researchers create their outputs, and then they own transfer to the publisher a “licence to publish”–they do not assign their copyright. Authors are often required to pay an “article processing charge” to publish in these journals. The published versions are immediately publicly accessible, free and reusable.
For more information about open access publishing models, check out the Australasian Open Access Strategy Group‘s website.