These are the rules that you need to alter to change the colours of the arrowheads. These two rules will not change the colours of the accordion background, so I am not sure what you are referring to.
If you used the recommended method to use custom css for the tabs, you can change the colours given in these two rules to customize the arrowheads for the accordion tabs in the normal state and when active.
As I cannot see your site I cannot give you any suggestions for what to change these values to.
How have you implemented the custom css for the tabs? Is is by using the method described on the plugin page ie by prevent the built-in styles from loading, copying the contents of the plugin's stylesheet into your child theme or custom styles plugin and making the changes to that? or did you make the changes by some other method.