Twelve colours for the maps with paired regions
This blanket represents a map of 24 paired regions; a couple of regions is identified by the same colour and, if we take any couple among the other 11, there's an element of the first couple that is adjacent of a region of the chosen one. We need 12 colours to colour the map in this way. In general, 12 colours are enough to colour every map with paired regions.
The image belongs to the sections...:
Bags, baskets, ecc (Anthology of images)
Colouring a surface (Topology)