Hi there!
I’m sorry to hear that you faced difficulties on changing the extension settings.

If I remember correctly, Firefox might have a bug that when you open color picker window, the options popup closes, resulting in color picker result not catched by the options page.

Could you please try to open extension settings in a separate tab and verify if the colors are saved correctly then? You can do it by clicking a button with arrow in the top right corner (it should say something like «Open in new tab» on hover). If I guessed the reason of your problem correctly, then you should be able to change tooltip colors on this page without any problem.