Long description

These are two identical rectangular grids, positioned side by side and linked by a double-headed arrow. Each grid consists of 168 small squares showing minor variations in colour in the blue-green range. In the top left-hand corner pale green fades into yellow, and in the bottom right-hand corner dark blue shades into purple. In the grid on the left a red line is drawn to include all the colours except those at the extreme edges of the rectangle. The area outlined in red is labelled ‘grue’. In the grid on the right, red dotted lines are drawn to include two overlapping areas: the one on the left is labelled ‘green’, and the one on the right is labelled ‘blue’.