Changing the colour and appearance of a Pine staircase can be done by either staining the wood with a liquid wood stain such as Manns Classic Pine Stain, Manns Trade Light Fast Wood Stain or Morrells Light Fast Wood Stain. These types of wood stains must be overcoated and sealed into the wood with a top coat of clear varnish or clear wood oil.
Coloured wood oils tend to look good but tinted varnishes obscure the grain to an extent. They are convenient and save having to stain separately but they don't enhance tthe grain in the same way.
See our full range of clear and coloured finishes for wooden stairs and staircases here.