Painting dark colours can be difficult. It is best if dark colours are transparent and warm. Dark colours usually should not be too close to the edges of the painting. Most artists mix their dark paints as this makes a more vibrant colour; they do not use black paint. The dark paints can be a red black, blue black, purple black and green black. Because I have difficulty painting black colours, I do use an Ivory black paint. I usually paint this over another colour, i.e. a warm blue. Once the black is dry, I add strokes of other colours such as a vibrant crimson and scarlet red. This provides me with a dark vibrant black. Also if you paint your dark colour beside a light colour, the value of the colours will be enhanced due to simultaneous contrast.