Gel or polyester filters. I usually use 4" Lee polyester filters or Wratten gels for lenses up to 105mm. You can get 6" gel filters for even bigger lenses.
You can also mount a gel holder inside on the rear element, even if there are no threads. There are some gel holders that use a spring or clamp mechanism. For some lenses that are impractical to filter in front, I've epoxied a square filter holder to the back of the lensboard.