There are an array of different models of Open Access publishing, and which option to choose can be confusing:
Subcription Model
This is the traditional model, whereby only those with a subscription can see the journal and the submitted research. This restricts who can see it.
Green
Govt institutions often push for this model of OPen access publishing. Green gets the pre print, - word doc format version of the eventual published (often in a repository like figshare)
Gold open access
This is free to read, but the cost of the process is borne by the researcher, who has to pay an article processing charge. This processing fee can be as high as thousands of dollars, and may be covered as part of the grant the researcher recieves. These journals may be hybrid journals, pubishing trad and open.
Diamond
This model results in published items that are both free to publish, free to read.