Failure of dried paint to obscure or 'hide' the surface to which it is applied.
If the substrate is significantly darker or is a patterned wallpaper, it should be primed before applying a topcoat. Use a top quality paint for better hiding and flow. Use quality tools, and use the recommended roller nap if rolling. Follow manufacturers recommendation on spread rate. If using tinted paint, use the correct tinting base. Where a low-hiding organic colour must be used, apply a primer first.
Source: The Paint Quality Institute