Hello Tony

Please share the page link here to check the issue. It can be possible by Custom CSS code.

Best regards

You are using a third-party template which has modified template.css. The colors are coming from the template.css file. You need to contact your template provider for assistance. Just for your information, I am giving you hints. If you use this CSS to Template Options -> Custom Code -> Custom CSS, then all overlay colors will be grey.

.uk-overlay-primary {
    background-color: grey !important;
}

Thanks for the hint Ofi - using the css, the background does indeed work now but it does NOT overlay the image - just a coloured background.

Cheers Tony

You are welcome. As I said, you need to contact your template provider. We can't really on third-party templates (even if it is made with Helix Ultimate). So, above CSS was just a hint for you. You need to figure out the style for your site.