To the non-folder, one of the most striking special effects that an origami artist can create is that of the color-change. I have had to go to as far as taking apart my models to show that only one sheet was used to create a two colored model (I guess that monochromatic papers abound). It is an effect that I try to incorporate into most of my models; it is almost mandatory for models of many subjects (such as a person holding something). The color layout of a subject is also one of my first considerations when I start creating. This is because it can be difficult to implement in the later stages of creating.
Almost surprisingly, I have yet to encounter another creator who sets color changes at such a high priority. One might also wonder why most models do not come out like a mulatto patchwork. The reason for this is that most origami appendages are sort of a symmetrical pleat. If you were to form a fan, as long as the number of sections remained even, the fan in its closed position would only reveal one color. Conversely, a fan with an odd number of pleats would reveal both sides of the paper. Most origami models begin by dividing the paper in half, which almost ensures that all of the appendages will have an even number of pleats. Consequently, creators do not have to worry about their models coming out anything other than monochromatic.
When a two toned model is desired, the creator often has to do something extra. Using our fan as a model, one solution would be to use an odd number of pleats. This solution can be difficult to implement, as most folders are used to working with even-pleated appendages. The way to circumvent this problem is to use a hemming technique. If you were to fold a third of your paper in, you would then have both colors revealed on one side of the paper. The paper can then be treated as if it were one layer; the resulting model will have half of its appendages revealing the alternate color.
This is not the typical way in which color changes are performed. The creator will often take a flap of one color, and then wrap layers from that flap around, so as to reveal the contrasting color. In order to facilitate this, the flap has to be reverse folded; this gives the flap an extra corner, so there is something to wrap around. As you might have guessed, this reverse folding changes the orientation of the flap (often by 180º), so it is certainly an important consideration.
An alternative way to colour change would be to crimp the flap at its base. If the crimp is wide enough, you can pull layers through the resulting gap. It might not be necessary to colour change both sides if a flap; in such instances your crimp can be narrower. This method retains the orientation of the flap, but it does dwarf its length. This is yet another consideration.
What I failed to mention in the above discussion is that these techniques can only be used on flaps that originate from either the perimeter of the paper, or the original corners. Such flaps contain raw edges that can be wrapped around. Even the simplest of models contain appendages from that other source, the middle portion of the paper. An example of such an appendage would be the top of a waterbomb base. The only way that I know of to change its color would be to sink it enough, so that the other color is revealed. This sort of procedure is difficult in many cases, and it is a radical reorientation of the original flap. If you are planning on doing this, you should have your model well planned out.
The way that I usually color change such flaps might sound radical. I leave the flap as is, and then I proceed to color change the rest of the model. An example of this can be found in the folding of my “Mosquito.” For this model, the wings originate from the middle of the paper, while the remainder of the appendages come from the perimeter of the square. For most of the folding sequence, the model appears white, as this will be the color of the wings. The last steps of the model consist of turning each of the appendages inside-out, revealing their color hidden within.
Alternate colored flaps can be impressive, but having colored patters on those flaps can be even more impressive. This can be a difficult problem to discuss, as there seems to be more solutions than there are problems. The one constant factor is that these methods consume a lot of paper. If you are planning on having an appendage with a fancy pattern on it, you may need a more than twice the length as originally anticipated.
I do have some tricks that I can share. To get spots on a flap, I like to think of each spot as being a separate appendage. When the appendage is flattened and turned inside-out, it will metamorphosize into a spot. Forming stripes is an extension of the crimping technique discussed earlier. If you were to take a bird-base flap an execute multiple crimps along it, color changes can be performed in-between sets of crimps. By color changing every other set of crimps, a stripe effect will emerge. John Montroll also has a clever way of obtaining stripes. He will form a fan-like appendage over the area in which stripes are desired. The fan is then spread out, staggering the raw edges. Miraculously, stripes will appear. Of course, such techniques can not be implemented on flaps originating from the middle of the paper.
If you are planning on incorporating color patterns in your model, it should be clear to see why planning ahead is necessary. You might have an efficient design idea for a model, but it might be difficult to have the color pattern desired if too many of the flaps originate from the middle. Quite often, such appendages are the same color as each other, so you should plan to have them be the same color in your final color configuration. Of course, in origami, anything is possible; I have seen many models with middle originating appendages that are different colors from each other. This of course, requires yet even more planning. Perhaps I ought to stick with simple monochromatic models.