Dark colours are richer when they are transparent and warm. I started out painting with watercolours and I learned to mix vibrant dark colours, i.e. pthalo green and crimson. I did not use black paint. When I started painting with acrylic paints, I found I could achieve black local colours best using black paint. (Local colour is the object’s colour, i.e. red apple, black coat.) I use Ivory Black paint over a blue underpainting to achieve a rich black as a local colour. I then add strokes of colours over the black, i.e. red, green. I do not use black paint to make shadows, cast shadows, shades or darker colours.