Lois - I've seen this technique several times recently, and actully performed it at a stamp club meeting. SUPER easy.
Select your image and two coordinating stamp pads; say, the shadow flower from shapes & shadows, and pretty in pink and pink passion.
Using your stamp, ink up the lighter of the two colors -- this is the IMPORTANT part. Be sure to ink the lighter color first!
Then, using the darker ink, take the stamp and ink the edges of the image only by rolling the stamp around the perimeter. This will ink up the edges of the image in the darker ink.
Stamp onto paper. The center of the image is the lighter ink, the outer edges the darker color.
Here's a sample from the gallery (thanks Babs!):
Gallery at Splitcoaststampers
I've seen this technique used with yellow, orange and pumpkins (our stamp club project), and it was very cool.
HTH!