Limewash
Limewash is slaked lime thinned with water, sometimes with earth pigment added. It is not a paint film sitting on a wall but a thin mineral layer that carbonates and binds into the surface underneath. That is why it can only be used on porous grounds such as lime plaster, brick, stone or earth, and why it fails on modern sealed surfaces.
Applied in several thin coats it produces a soft, slightly cloudy depth that is difficult to imitate with a flat emulsion. Each coat is nearly transparent, so the finished colour is built up from layers and shifts as the light moves across it. The effect is strongest with the earth pigments that limewash will accept: ochres, umbers, red earths and a limited range of greens.
Its practical virtue is permeability. A limewashed wall lets moisture move through it, which matters a great deal in solid-walled buildings without a damp-proof course, where an impermeable coating traps water in the fabric.
Its practical drawback is dusting. A fresh limewash will mark clothing until it has fully carbonated, and even then it is not a wipeable surface, so it belongs on walls and ceilings rather than in circulation zones at hand height.