Recipients visit the reply page of your event and choose one of the reply options.
The first option WILL ATTEND marks a recipient as GREEN.
The 2nd option WILL NOT ATTEND marks a recipient as RED.
You can also edit a the reply status of a recipient manually and set a recipient to GREEN or RED yourself.
To edit manually please go to the grey drop down in each recipient row and click on "EDIT REPLY".
Here you can choose GREEN or RED.