ImageSorter is a free tool which organises and sorts images by colour.
We pointed the program at a folder, and it quickly read 270 images, presented them in thumbnails, and sorted them into colour and layout.
This worked surprisingly well. Cityscapes, faces, forest scenes, sunsets and more were generally grouped together, making it easy to spot duplicates and find similar images.
Double-clicking any image opens it in your standard viewer, and there are tools to copy, move or delete selected images.
ImageSorter is old, and a beta, and unsupported, and does have some issues (a sidebar with various search filters appears to be almost entirely useless). But its key "sort by colour/ similarity" tool worked well for us, and justifies the download all on its own.