Portraitists' paints refers to the type of paint used by portraitists, which is typically oil paint. Oil paint is a slow-drying paint made from pigments suspended in a drying oil, such as linseed oil. It has been used for centuries and is still popular today due to its ability to create a wide range of colors and textures. Oil paints are also known for their durability and longevity, making them ideal for creating portraits that will last for generations.