This can depend on which part of the window frame is to be filled and what type of finish the window frame will be coated with once repaired. Some wood fillers are only suitable for interior use while others are formulated for exterior use. If the window frame is to be painted after repair it doesn't matter if you use a filler that has a solid opaque colour as it will be covered by the paint. If the window is being treated with a semi-translucent coating system such as Sikkens or Sadolin, you'll need the repaired area to look more like wood so we recommend Fiddes Wood Filler Gel. These types of filler are mixed with sanding dust, (it is preferable to use dust from the wood that is to be filled which helps to produce a better colour match), to produce a wood filler paste that is essentially half made of wood and therefore will absorb the wood finishing product in a more similar way to the wood surrounding it. Once coated with a semi-translucent finish, the repaired area will be less visible than with a standard opaque wood filler.
See our full range of fillers suitable for windows, frames and sills.