Like painting a house that is dark brown (or black, navy, etc.) using white, or light color, may require two coats or a base coat.

The same holds true for screenprinting. When printing on dark colors of garments a separate screen of white under base is highly recommended. This "flash plate" keeps the colors from blending with the dark background which produces a muted image color.

If white ink is not one of your design colors, then you need to ADD this white screen to the total colors in your design. This will increase your costs. Also the graphics for printing on dark colors are bolder and without fine details since a heavy layer of light colored ink must be applied in order to cover the dark substrate and prevent bleeding.