To make a successful wash:
Get some hardware store mineral spirits. Buy it by the gallon at a big box store.
Go to Hobby Lobby or Michaels and buy some Winsor & Newton oil paints. There are different grades. Buy what you can afford. They are all good. Get Burnt Umber, Ivory Black, Titanium White, Yellow Ochre, Sap Green, Cadmium Lemon, and Prussian Blue. This will also have you set up to do dot streaking in the future.
But a set of cheap stencil brushes.
Buy a package of disposable medicine cups.
Buy a box of toothpicks, if you don’t already have a lifetime supply.
Fill the medicine cup with mineral spirits. Open the burnt umber and squeeze out an aspirin sized amount of oil paint. Slice it into the medicine cup with a toothpick. Mix well until all the oil paint is dissolved into the mineral spirit. You now have a perfect burnt umber wash. Total cost - about half of one penny. Compare that to what AK charges. Apply the wash. Let it dry. If you get tide marks or pooling, scrub the stains away with the stencil brushes dampened in mineral spirits. Reapply the wash until you get the depth of effect you are looking for. You can switch to black but it has much starker results, sometimes desired. Cut the amount of oil paint in half and switch colors to make filters and bathe them over an area to shift the color. Make sure you allow each wash/filter to dry several hours or overnight. Oils have the advantage of having a very long working time and give you a nuance and subtlety that enamels or acrylics can’t. Give this a try. It’s really that easy.