Skinner blends, named after Judith Skinner, are usually made from two triangles placed together to make a rectangle.  You then fold them color to color and pass them through the pasta machine anywhere form 20 to 30 times.  Color to color,  say you are using white and red, once you put the two triangles together you fold the sheet in half, keeping the white edge on the same side.  So you have a sheet with red on one side and white on the other.  (It can be confusing!)  As you pass the blend through the pasta machine the colors will blend into the middle to create a blend that goes from white to red with many different shades in the middle.

 You can also do blends with three or more colors.  The trick to doing any skinner blend is to keep the edges the same color when you fold the sheet in half.