As the paints are developed by Axalta you can not create new ones from scratch, but you can tweak the colors and make adjustments to roughness and reflection.
Color Tweaker Advanced option for fine adjustments to the way the paint behaves when viewed at certain angles.
Roughness This adds microscopic levels of imperfections to the surface of the material when the values are increased. When this is set to 0, a material will appear perfectly smooth and polished. When the value is increased, the material will appear more matte as light is diffused across the surface.
Refraction Index This slider provides controls the reflection on the surface. Increase the value to get more reflection intensity.
Samples A low samples setting (8 or lower) will tend to make the surface look more noisy, which will give a more imperfect and rough look. As you increase the value, the noise will smooth out and provide a more evenly distributed roughness.