I want to darken a PNG image in GIMP. I open it and choose Colors | Brightness-Contrast. The application responds with the error message - "brightness-contrast does not operate on indexed layers".

How can I darken the image?

1 Answer 1


Whenever we have an indexed image source (e.g. 256 color palette) we will not be able to adjust brightness and contrast with Gimp.

To still do so we will have to convert the image mode to RGB first (Image > Mode > RGB), perform our adjustments and as a last step reduce the colors to indexed if needed.

Color and brightness adjustments of images with a reduced color depth may however not give us good result.

  • 2
    Perfectly correct - up to GIMP 2.8, which is the current stable. One of the most notable features of the current unstable version (which will eventually be GIMP 2.10) is the ability of performing any operation with an image even if it remains in indexed mode.
    – jsbueno
    Apr 22, 2014 at 4:08
  • I wish I knew this before I spent ~5 minutes doing that carefully-placed lasso select. =P
    – user75628
    Sep 5, 2016 at 0:48

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