Further adventures in creating a custom math-themed jigsaw puzzle
nibblestew.blogspot.com
nibblestew.blogspot.com
https://www.vistaprint.co.uk/photo-gifts/custom-jigsaw-puzzl...
https://www.hoferfoto.si/sestavljanka
They even make wooden ones ( https://www.hoferfoto.si/sestavljanka-iz-lesa )
(not an ad, just showing that it's a cheap service)
So starting with solid dark grey, ending at solid light grey and in between doing a human visible pattern of on/off that varied along the length?