In the last century, synthetic polymer paints were developed. The first was acrylic paint, which was introduced in the 1940s. Acrylic paint is opaque. It’s very easy to use. You can use mediums and water to change the surface.
In this image, the scene was live. People were moving around. This was not a pose.
I used small brushes. The portrait is of a senior woman wearing a red singlet, and with her hair stacked. On a table in front of her are an art book, her glasses, her charcoal drawing and some translucent boxes of art supplies. Behind her on one side is a blue plastic hamper, and a black wooden stool. Above it is a table full of art supplies. People are coming and going. In the background are blue cupboards.