Using a change of contrast for focus and other states is a technique to differentiate the states. This is the basis for G195: Using an author-supplied, highly visible focus indicator, and more techniques are being added.
-In combination with 2.4.7 Focus Visible, the visual focus indicator for a component must have sufficient contrast against the adjacent background when the component is focused, except where the appearance of the component is determined by the user agent and not modified by the author. If the focus state relies on a change of color (e.g., changing only the background color of a button), then changing from one color to another that has at least a 3:1 contrast ratio with the previous state of the control is a method for meeting the Focus visible criteria.
+In combination with 2.4.7 Focus Visible, the visual focus indicator for a component must have sufficient contrast against the adjacent background when the component is focused, except where the appearance of the component is determined by the user agent and not modified by the author.
+Note that this Success Criterion does not directly compare the focused and unfocused states of a control - if the focus state relies on a change of color (e.g., changing only the background color of a button), this Success Criterion does not define any requirement for the difference in contrast between the two states.