If your using an oil wash or even acrylic, there’s no set ratio. You have to experiment with different dilutions to see what you want ( a thick, weathered wash or a light panel line accent). I use watercolors for my washes since they remove easy. I use a pea sized drop of paint and add water till I have what I want. Best advice…experiment to see what you like and make note of your mix ratio. Remember, the washes will dry lighter than they are when wet. Don’t worry if you feel it’s too dark when first applied.
I mix my washes from pastels, water and some soap. Works wonders for me. Then you just wipe it off with a moist cloth or paper. Perfect when adding panel lines or depth in bulletholes and cuts.
But I suppose any of the three you mentioned would work. The soap only acts as a ‘retarder’ so that the mix can fill in lines and gaps easier, and stay there when you brush the excess off with a moist cloth or paper.
I would image acrylics would be alright as long as you put a FLAT coat on if your doing the pastel thing since they wont stick to a glossy surface.
Or certainly mine never had.LOL
I use acrylic washes over enamels that have a coat of future.
I use an eye dropper kind of thing, and put 5 drops water, 2 drops liquid dish soap and 1 drop paint in a small bottle cap (like off a Coke bottle). I stir it with a toothpick.
I test it in a place or two (it dries quickly) and if it rubs off fine, I go with it. If the wash has trouble coming off (I use Qtips), I add another drop of soap.