Important color images can be preserved with color separation technique, this is making 3 contact copies on BW copy film, each copy made with a different basic light color, I guess that depending on if it is a positive or negative image CMY or RGB can be used.
In fact Hollywood color movies are even today stored in this way, including movies shot digitally. Kodak 2237 and 2238 films are used for this: "This black & white recorder film is intended for making archival separations from color digital masters", Kodak says.
For color sheets that technique is also possible with regular fine grained film. Another way is refrigeration (40°F (4.4°C) and 27% RH, for example), that slows chemical changes, and I guess that concurrent inert gas storage may be also good.
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