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I am in the process of digitizing a large number of slides. All of them have a heavy magenta cast to them.I have been using photoshop to attempt correction but with limited success. The ones that I have the most trouble with were taken inside a mine using flash and sometimes multiple flash but the cast is also present in outside shots without flash. The slides were taken in 1973 through 1975. Use of Auto-Color in Photoshop does not appear to help.

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    Sample image would be helpful. The color cast might not be "pure". The images may have degraded unevenly, and it seems you're dealing with a mixed-lighting scenario (flash + ambient). – xiota Aug 3 '19 at 3:21
  • Would also be helpful if you describe your method of "digitizing". Are you scanning them? Photographing them with a slide duplicator? Using some other means? – xiota Aug 3 '19 at 3:38
  • I agree with @xiota. FWIW, generally green is opposite magenta on the color wheel, so that would be the first thing to try. – user1118321 Aug 3 '19 at 4:11
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If your images simply had a magenta cast, then simply adding in some green would balance it out and you could be on your way. Unfortunately, you are not so lucky.

Your images are magenta in color because the other dye layers in the film have broken down. You have magenta information but have lost the cyan and yellow. Simply adding green will not suffice.

In a nutshell, you will need to adjust the RGB color layers separately and will be bumping up the amount of blue and green by quite a bit. You may need to reduce red. The process is given in detail here: https://www.scantips.com/color.html

Once done with one, you could apply the same adjustments in batch to the rest. However, this would likely only give you a start, as there is no guarantee that each slide has aged the same.

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A color cast is neutralized by its complement (opposite color).

Green <> Magenta

Blue <> Yellow

Red <> Cyan

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