Hello, @Kelgor,
Thanks for the clarification. But sorry to say, that the add to cart button has the same class if you modify the color from the theme settings, or with the help of custom CSS code it will change the color completely for the available products and for the out-of-stock products as well. So the changes you require are only possible with the help of customization and we cannot cover customization issues in our support policy.
Thanks for your understanding.
Regards 8Themes Team.