(Completely rewritten version.)
I'm a writer doing a book for an overseas publisher. My deliverables for them are .doc files for the text and .png files for the graphics. I have no control over print production processes.
The publisher tells me that my .png files are "too light" and will cause problems during printing. (I've been told this doesn't make sense; nothing I can do about that.) They've asked me to do darker versions. Since these are screen captures of web applications, the only way to make them darker is to edit the files. I've tried editing them using the Brightness/Contrast tool in GIMP, but this introduces weird color distortions. There's presumably some simple adjustment that will make these images look darker, but I lack the expertise to find it.