Hi,
please go to Dashboard/Revolution slider. Click on slider that you need and then on Link & SEO tab. First input is Class, so add there some class like we did in picture in attachment. After that open your style.css file
(you can find it here \wp-content\themes\revelance\style.css)
and in the end of that file paste this code:
.dark-bg .tp-bgimg.defaultimg {
background: rgba(200, 54, 54, 0.9) !important;
}
You set color that you like, we set red just for example.
Kind regards