A wash is just a thinned out pigment (can be purpose bought or DIY) . Also known as an ink in some corners. These are brushed on generously so that the pigment settles into the crevices to form a natural shading.
Your best bet is to search on YouTube “Painting 28mm xyz” as there will be 999 people talking about painting very precise troop types and they normally give the paints they use.
