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I have some images from a Nikon SLR that I have converted to grayscale, however the thumbnails in Windows Explorer are in color. Is the thumbnail information stored in the file header, or is the color information still there, and if so can it be restored?

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    What file format are the images? – osullic May 27 '15 at 20:41
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    Did you convert them to grayscale after the thumbnail was created? What version of Windows? Windows stores the images for thumbnail previews in cache memory. When you clear or refresh the thumbnail cache the thumbnails will be updated. The thumbnail image is just a small image so it won't be the full image. – A K May 27 '15 at 23:53
  • If it's a cache, renaming the file would help. OTOH if you rename it in the same view that's showing the thumbnail it will know to reassociate it, too. So rename it on the command line. It would be odd that a program saving a jpeg file (the edited file needs to be totally re-encoded) would leave an old embedded thumbnail. – JDługosz May 28 '15 at 2:13
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    You never said jpeg, so maybe we're way off. Please update the question with details on how you converted the files: what program, how saved, and wat type of files were read and written? – JDługosz May 28 '15 at 2:16
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I assume you're speaking about JPEG (.jpg) files.

No, the colour information can't be restored. If the image was converted to greyscale it doesn't contain the original RGB data. Some programs just fail/omit updating included thumbnails in JPEG files. You can sometimes also see the full image prior cropping in a thumbnail which can be pretty undesirable in some cases.

Try to use a different software to edit your images to check if this is the problem.

Also the thumbnail database (thumbs.db, IIRC) which is a hidden file inside a folder might contain invalid/obsolete thumbnails. You might try to delete it and let it be rebuilt again.

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It usually depends on which software program you're using. Some software does "destructive" editing, where it actually changes the file to greyscale and permanently removes the colour information (programs like paint, photoshop (if you don't use layers or if you save as a .jpg, .gif, etc.) ). Other programs, like Lightroom put a digital layer "on top" of your image. Unfortunately, the thumbnails in windows won't reflect your edits until you "print" the image (essentially creating a completely new file that looks just like the edited image in the program.

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