As I was making this gallery, I realized that many of my favorite models, complex or simple, involve some sort of color change. I could argue that it's more challenging to utilize both sides of the paper, but I don't think that's the reason. They just look neat.

Wasp by KASAHARA Kunihiko, Creative Origami
Yoda by Dr. Stephen O'Hanlon
Black & White bird by me. It's a fairly common standing crane (meaning I have no idea where I got it from). I reversed one flap of the bird base to make the color change.