Many types of butterfly koi have distincive irregular patches of color on their bodies. To make the most common patterns, the deep orange or red color on silver or white bodies, shape your koi body and add the patches once the body is shaped.
To create the patches, roll out a very thin layer of the color you want to use for the patch color. Gently tear off small bits from this thin layer with the head of a pin, and set them in place on the surface of your polymer clay koi body.
To blend the patches even with the rest of the koi body, use a medium stiff brush to gently push the patch into the koi body with a 'pouncing' or up and down motion. When the patch is set into the body, use the brush to gently blend the edges so the patch and the body have the same smooth surface. You can use medium stiff bristle brushes to create an allover textured 'scale' pattern on your fish if you wish.
Other colored patterns on koi can be made by partially blending translucent and white or pearl clay with colors and making fish which have a partially marbelled or swirled color pattern. Check reference photos to see the colors of real fish for examples.