In my naïve view, I think of histograms and colour pickers as measurement tools/devices.
I think normally I’d be interested in the following measurements:
- input or output of the currently selected module, in the respective colour space (normally, the working space, except for modules that perform colour space conversion; but I think normally one would ignore those, as it’s the processing modules we’re interested in). Input is, of course, always the output of some other module, so this can be simplified to output of modules only. This is like measuring the signal at some point between stages of an amplifier or whatever;
- output after the output colour space conversion (the data that will end up in my exported file, sans compression artefacts). That is like measuring the final signal (using a strictly resistive load - but I don’t know enough about electrical engineering, so I’ll stop).
Involving the display profile makes little sense. One rarely judges amplifiers by placing a microphone in front of the speakers.