Add option for multi-colored backgrounds

[tokee]

Re-locating an image after forus has changed can be a challenge for at collection og 10,000+ images. One way to help could be to divide the overall collage into areas of different colors. Palettes suitable for color blind people can be found at: https://davidmathlogic.com/colorblind/ - the one by IBM does not look bad: #648fff #785ef0 #dc267f #fe6100 #ffb000.