using Photolab - Batch corrections - Curve

[Ofnuts]

Between Rich's  grayscale failure and the OP's error message that happens when trying to tweak a color channel, it seems the script only applies to full-RGB images. A possible fix (untested) that skips the color channels if the image isn't RGB:

   
 
Used a decent text editor for the fix (not notepad/wordpad, that usually messes the indentation).

Note that the script will still possibly fail on color-indexed images (GIF and some PNG).