I'd agree with the previous posters - slap some yoghurt or whatever on it.
Urine has been suggested too. Soil can have a high organic content, and you
can see this by getting some, shaking it in a jar of water and watching how
soil separates out as it settles. I'd at least avoid putting the organic
Another suggestion.: I notice how nearly everything I have at ground level
takes on something of the colour of the local ( clay ) soil, so why don't
up your mortar the normal way, use it to build your structure, and then when
has gone off, simply rub wet soil into it? That'll tone it down right away,
won't wash off easily.