Two photographs with differing geometry (made with a side-shift in camera position) are colored with complimentary colors which are seen as black through filters of the opposite colors. The filters are typically red and green but sometimes red and blue.
Example
The red and green images of a wooden frog were made with a digital camera and colorized in Photoshop. Layers were then pasted to a third frame to overlay the images. Viewing with red/green glasses gives the full 3D effect in more or less natural colors.
The technique is sometimes used in magazine illustrations and for children's novelty items.
Mars (Surface feature)
Phobos (The larger moon)
A large collection is on the web at....
http://3d.usachoice.net/gallery.htm