I have seen your screenshot and also visit your site. You are pointing to the custom button which you added with wp-bakery builder to display a popup. So you need to edit the popup element where you used and change its color directly.
I visited your site, as you are using Wp-Bakery code inside that product short description area to display the popup button. And Wp-bakery is not accessible in short description area. So we first have to copy that code to some other dummy page and edit it with Wp-Bakery editor and then define a custom CSS class in column and then put back to short description area.
Then I add custom CSS code according to that class in global custom CSS area under theme settings >> Custom CSS, to get the result in that product page: