Hi,

Did you mean the BG color? The BG color will be the same as ratio.
I suggest you set it the same as page BG color.

Or you need to set the CSS for each image manually.
For the portrait images (if image ratio is 1:2, width is 50%) by custom CSS(“Appearance > Customize: Additional CSS”) (with the Post ID) :

section[data-postid="101"] .grid-item-con:after,
section[data-postid="102"] .grid-item-con:after { 
    width: 50%;  
    left: 50%;
    transform: translateX(-50%); 
}

For the landscape images (if image ratio is 2:1, height is 50%) :

section[data-postid="101"] .grid-item-con:after,
section[data-postid="102"] .grid-item-con:after { 
    height: 50%;  
    top: 50%;
    transform: translateY(-50%); 
}

How to find page/post id? https://doc.seatheme.net/wordpress-faqs/custom/how-to-find-wordpress-post-page-id/

Regards!
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