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?
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.
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.