There are a variety of materials that artists can use for highlights. Some common options include white paint, Gouache, and white pastel. Each material has its own unique properties that can affect the look of the final piece.
White paint is perhaps the most versatile option when it comes to highlights. It can be used with a variety of other colors to create different effects. Gouache is another popular choice for highlights as it is opaque and provides a smooth finish. White pastel can also be used for highlights, but it tends to be more difficult to control than the other two options.