Most washes have three basic components in common:
The wash medium, that makes up the bulk of the wash (usually water);
The pigment (paint, ink, etc.);
And a surfactant (something that reduces the surface tension of water). This is what allows the wash to settle down into the crevices of the model, rather than “pooling” on the surface.
For years, painters used a basic wash of water + paint + dish soap to achieve this. Many still use just that, and it can work well.
Found here.
https://modelrailwayengineer.com/model-making-101-making-washes/
Hope this helps