Filters are a shade of the base coat applied after the base coat.  You can make your own btw, with oils and mineral spirits.   I don't see a need to buy filters or washes if you have a stable of basic oil colors, except for colors difficult to create (such as sandgelb).

If you are doing a 43 and after panzer in Dunkelgelb nach munster, use yellow ochre and white artist oils and just a sliver  of prussian blue (less than one percent if that).  You want a shade slightly lighter than the basecoat.   Mix it heavily with mineral spirits or more accurately mix mineral spirits with a small amount of the oil mix, to get a translucent fluid tinted with the color you are after.