Pigment from soils?
Even to this day many of our paint pigments are mined this way. Red/yellow ochre, umber, sienna.
If what you are asking is the dirt in question geologically speaking a soil? Sometimes, sometimes not. It can be a sediment or a regolith too. But in the more general laymen sense callig any dirt from the ground a soil is not too mistaken.
Now i’m just recognising this might be a language issue: watercolour paints are a type of paint to paint with on paper using a brush dipped in water. If you ever seen kids paint with brushes and paper most likely they were using watercolour paint.