Sangwine, S. J., 'Colour image edge detector based on quaternion convolution', Electronics Letters, 34, (10), May 14 1998, 969-971.
The first image appears to be monochrome at first sight, but on closer inspection, will be found to have coloured edges as described in the paper.
All three files can be downloaded in PNG (portable network graphics)
format for viewing in an image viewer if you have one (especially useful
for zooming in):
Figure 1 (359kB)
Figure 2 (3kB)
Figure 3 (4kB)