Well, I have not seen the pictures that you are talking about...but I can tell you that it would be a pretty easy trick to pull off using a program such as Adobe Photoshop.
Basically though, most Paint style programs will allow you to replace one color with another (or to use a Paint Can type tool whereby you fill an entire area with a new color).
Play around with the Paint program that comes with Windows and you might find that its basic features allow you to produce this effect...
Mike