DC Motor Winding Insulation Material
Wire used for motor windings, transformers and other electro-magnetic applications is called magnet wire. Magnet wire insulation is similar to polyurethane varnish. However is is developed specifically for electrical insulation use. As you have observed, the insulation is quite thin yet able to withstand the normal operating temperature and voltage required for motor windings. It is not completely transparent. It is sometimes colored green or other colors that do not look like copper. The most common color is similar to copper. There doesn't seem to be any particular reason to use other colors.