I have walls that whitewashed using water emulsion paint, and bed linen of dark blue color. Bed linen pigment is unstable, and always paints everything it touches with the same blue-ish color. I tried to wash it in big temperature to remove that pigment, but nothing worked, and when I left it on the heating battery on the wall to dry, it has painted the wall it has been touching . Now when I try to remove that blue paint with wet rag, it looks like removing some of whitewash on the wall, and leaves some spots different from the other surface of the wall
It's far easier to paint over a large stain than to remove the stain, which may well have penetrated into the "whitewash" on the wall. One would have to remove the paint to remove the stain.
After painting, most people keep leftover paint for such patching, and perhaps the paint type and custom color identifiers. If renting, ask the building superintendent for some paint to match. It's nothing to be embarrassed about, as scratches and stains occur often.