That's by design with the software. Product attribute selection can drive thumbnails, but thumbnails do not drive attribute selection. When you hit the main product URL, it's a compilation of all images, which is what you're describing. When combination (Color) specific images are set, if user selects an color attribute, it will filter the images down. For this reason, I use this module on my site. It allows me to split the product by color and present each in my category page separately: https://addons.prestashop.com/en/product-variants/20451-products-by-attributes-display-combinations-attributes.html.