When all else fails, use Separation Masters - a backup of the negative made before copying which consists of the three colours that make up a full colour picture separated onto three strips of black & white film. Pollyanna & Spartacus were restored by recombining the seps because the negatives were faded beyond use.
It's not perfect, because this adds more grain, misalignment can cause rainbowed edges, and it's expensive, but like I said, when all else fails.