Buttons are used to provide a physical affordance to actions across the Ubuntu interface.
Positive – Primary
This type of button is used in green to trigger a positive action (e.g. OK, New, Call etc.)
Negative – Primary
This type of button is used in red to trigger a negative and irreversible action (e.g. Delete, Archive, etc.)
Neutral – Primary
This type of button is used to trigger a neutral action (e.g. Cancel, etc.)
Stroke buttons – Secondary
The outline button is used in conjunction with primary buttons to indicate a less important or frequently used action.
Note: it might have a positive/negative association and therefore be red or green.
A combo button is used to provide a large set of actions in a compact space. It combines a default action that is activated by tapping the main target area. A popover will appear with secondary actions when the user taps the target area on the right.