Added the ADJUST_COLORS option to adjust each tile's color balance to match the target fragment better. Produces better visual quality with fewer tiles, especially when there are few source tile images.
Added a palette of plain color PNG images in the images/colors directory for quick experiments, and for a cool pixel art effect. Also added the script extras/gen_colors_rgb.py that generates those.
Added optional reporting of three statistical metrics for the similarity of the target image and the mosaic: MSE, structural similarity index, perceptual hash difference.
Feel free to pick only those parts that you like.