Thermal Paper has four pieces of paper

The top layer is white. This is the layer that your image will be transferred onto.

Second Layer is not connected and is a milky white. Typically this layer is thrown away.

The third Layer is Carbon (purple/blue paper). By pressing on this layer the purple/blue coating is transferred to the upper sheet.

The fourth bottom sheet is yellow and holds the original in place when using a stencil copy machine.