Step 1: Review the research on the Watercolor Illusion here.
Step 2: Experiment by creating several different lines in a dark color and then flanking it with a slightly darker color on a white piece of paper. Does it make it look like the inside between the lines makes a different color? Use different lines, spaces and background colors to vary the experiment.
Step 3: Explore a variety of old maps and cartography that used purple and orange to delineate geographical areas. Create your own map using this technique.