A washable paint is washable, not indestructible. The usual sequence is a mark, then a vigorous scrub with whatever is under the sink, then a permanently shiny patch that is more obvious than the mark ever was.
Start with the gentlest thing that works
Warm water and a soft cloth, wrung out nearly dry. Blot rather than rub, and work from the outside of the mark inwards so you do not spread it.
If that fails, a drop of washing-up liquid in the water. Only then anything stronger, and never an abrasive cream or a scouring pad on an emulsion.
Know what your finish can take
Silk and satin will take a damp cloth routinely. Matt will not — it is the nature of a flat film, not a fault in the paint, and rubbing it will burnish a shiny mark into the surface.
On a matt wall, touching in with a little of the original paint is usually a better answer than cleaning.
Touching in properly
Keep a labelled jar of each colour when you finish a job. Use a small brush, feather the edges out, and apply it thinly.
A touch-in will always be slightly different from a wall that has had a year of light on it. Feathering makes the transition gradual enough that the eye does not catch it; a hard-edged dab never blends.
When to stop cleaning and repaint
If a wall needs cleaning more than once or twice a year, the finish is wrong for the room rather than the cleaning being wrong.
A hallway in matt will keep asking for attention. The same hallway in silk will not.
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