1) add a new field into 4images_images table, lets call it "image_color". You can execute in your mysql manager (phpmyadmin):
ALTER TABLE `4images_images` ADD `image_color` VARCHAR( 20 ) NOT NULL
2) in includes/db_fields_definitions.php add:
$additional_image_fields['image_color'] = array("Image Color", "text", 0);
3) edit your images and you should see new field "Image Color". You can either use CSS class name or HEX color code or color name, whatever.
4) in details.html template use something like this (as an example Image Color contains CSS class name):
{if image_color}<span class="{image_color}">{endif image_color}{image_name}{if image_color}</span>{endif image_color}