To achieve the progressive loading of images, a background color can be used, which can be assigned before loading is performed and it will be shown during the loading phase of the web page. It could be an average of the image color, the predominant color, the same background color of the image, a background image or a color you like. This is an example of how to integrate it into an existing WordPress theme with filters. It is also possible to alter the IPTC metadata of the uploaded image and magically put a color value to the attachment metadata.
I used "wp_read_image_metadata" and "wp_generate_attachment_metadata" hooks to add a color value with the IPTC Instructions field and save a post metadata of the attachment.
/**
* IPTC metadata - Instru