Definition from JISC’s website:

Gold open access

Some journals contain only open access articles whereas others, known as hybrid journals, may offer a mix of open and subscription content. In both cases, there are costs associated with publishing which need to be met.

These costs, usually paid through article processing charges (APCs)¹, may be paid by authors or subsidised by a third party such as a funding council.  The article is made available immediately. This route is known as gold open access. 

Green open access

Green open access involves publishing in a traditional subscription journal as usual, but articles are also 'self-archived' in a repository (institutional or external subject-based repository) and usually made available after an embargo period set by the publisher. No charges are paid.  

These definitions are not fixed, so you might find slightly different explanations on different websites.

Actually there is a sort of continuum which goes from maximum openness, that is:

• Free readership rights to all articles immediately upon publication
• Author may post any version to any repository or website
• Journals make copies of articles automatically available in trusted third-party repositories (e.g., PubMed Central) immediately upon publication
• Article full text, metadata, citations, & data, including supplementary data, provided in community machine-readable standard formats through a community standard API or protocol

To a situation when access is possible only for a fee, the publisher detains copyright, the author is not allowed to post any version on other websites or repositories, and articles and metadata are not machine-readable in any way.

Of course there are many different positions in between.

Our University supports Green Open Access.

However, in case Gold Open Access is necessary, the Library is actively negotiating transformative agreements with several publishers, which result in APC discounts for authors.

¹An article processing charge (APC), also known as a publication fee, is a fee which is sometimes charged to authors to make a work available open access in either an open access journal or hybrid journal. This fee is usually paid by an author's institution or research funder rather than by the authors themselves. [by wikipedia]