Olympus cameras have an option in the auto white balance that says ‘keep warm color’ on/off or something similar.
If you turn it off , shots like this will be neutral . But sometimes you want the warmth (to capture a sunny day as a warm sunny day for example :)).
The Olympus editing tool (om workspace its called these days I believe ?) doesn’t work nice in my opinion , but it allows you to pick the other AWB option for raw files I believe .
What others said , using color calibration is the most correct way , but any way you get results is ok, don’t worry :).
The warning about white balance and color calibration is because they both attempt to do the same. If it gives a result you like : nothing wrong with that . If you like the white balance control more , turn of color calibration and use white balance to pick a part of white.
But color calibration can do the same.
Use the picker in there to select a large part of the wall, And it should make the wall white.
As you said , it isn’t really white. So use the chroma slider in color calibration after picking it to undo the effect slightly , to your liking.
Color calibration also allows you to target something else than white / gray.
You could take a photo of the same wall in normal daylight lighting. Open that in darktable, and use color calibrate to select the wall but use it as a target. (It’s in one of the collapsed sections in the color calibration module ). Now open this image (the yellow tinted one ), and select the wall. It will now turn it your previously selected colour , instead of white / neutral .
For the record : I THINK you want to do the correction on the whole image . It’s not just the wall that’s off , this is lighting conditions so everything is tinted.
So in case there are people in the shot, those will probably also be way too yellow / warm .