This color choice only covers the knees or elbows of the limb, this allows you to push limbs together to make it seem like a solid color and a part of the body. The best limb to use is the slackwrist arm, as this makes it flow more smoothly with the normal body, however it works with any limb. The screenshot below shows how you have to push the limbs together to get this effect.
Shape the limbs anyway you want to, however make sure they are sized correctly so they flow together. The result should look somewhat like this:
Part of the body is the base color, but the other part is the detail color shaped to look like it's part of the body color.