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?
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.
Source : Link , Question Author : Synesso , Answer Author : Takkat