Learn Something New Everyday
The easiest way to do this digitally is to use the channel mixer in your favorite photo editing software to make one photo the red channel (set green and blue channels to zero, leaving red at +100), one the green channel, and the last a blue channel. After you've chosen your base layer, you copy and paste the other two photos on top of it, then set those top two layers to difference blending. After that you can nudge the top two layers around to align them better, if necessary.
This is a fun effect to play around with! See this Flickr thread for more information and examples: <a href=”http://www.flickr.com/groups/technique/discuss/72157602020995316/” >www.flickr.com/groups/technique/discuss/72157602020995316/</a>
BTW, as a quick and dirty test, I took these photos with my iPhone, then post processed on my laptop. I wish I could've edited them on the iPhone, but AFAIK the existing photo editors (Photogene, Iris, PhotoForge) don't have a channel mixer feature… Please correct me if I'm wrong!
P.S. As a bonus learning experience I used Lab color processing to make the colors pop a bit more. I used the steps discussed in this other Flickr thread: <a href=”http://www.flickr.com/groups/technique/discuss/72057594133492563/” >www.flickr.com/groups/technique/discuss/72057594133492563/</a>
Have fun experimenting!