Although it uses 31 colors, it's not an OCS/ECS picture because of the use of non-12-bit-color values. The colors can only be displayed correctly on an Amiga AGA chipset in 5 bitplanes.

This can come as a really bad surprise to the gfx artist if you want to include such gfx in an OCS production.
Due to the lower color resolution, palettes suddenly have duplicate entries and gradients are all messed up.
If you're painting on an AGA machine, select "Original chipset" in the boot menu. Then you have the 12 bit colors in the paint programs.