There are gelatin filters designed specifically for lenses. I have a full set from "the old days" from my 4x5 camera studio kit. I mounted them with a gel filter holder. They allegedly cause less distortion than traditional glass filters. They can be mounted in front of the lens, or inside the camera, at the rear of the lens.
Oddly, for ultra-critical work with the Nikon D2x, the use of a Wratten 40cc gelatin filter is recommended by one of the most technically-critical people I know; he claims it perfectly balances out all the spectral response of the RGB channels of the D2x's sensor. The D2x had,at its inception, one of the most technically accurate color responses, and was and still is used quite a bit by people who specialize in copying paintings and other art works, and Iliah has determined that a 40cc magenta gelatin is the perfect channel equalizing filtration for that camera's sensor when the *utmost* color fidelity is needed.
These camera lens gelatin-style filters used to be sold by Kodak in 3x4 inch "packets", sort of an envelop-style carrier. Complete sets like the one I got 25+ years ago often came in a hard, hinged case with little plastic leaves, likie a miniature photo album. With color slide film, this was how you made minor color corrections, using 10,15,20,30,and 40cc gelatins.