This is what I did.
Used a small flat plastic lid from some type of food product. I bought three colors. White, black and a grey. I took a
tiny bit of color (little goes a long very long way) and blended the colors to the shade I thought would look good on the area I wanted to wash.
Took a little bit of distilled water and dish soap and dropped it on the color then mixed until I created a puddle of solid mass and thin wash. You could probably call this a sludge. Brushed it on, set it aside until it dried then used a q-tip slightly moistened with water to clean it up. It does not take long to dry. If you don't like the results, wash the part off and try again.
I have read where some modelers use vinegar but saw no need with the soap and distilled water. Also, after a time the sludge mix dries on the lid but you can get it wet again by simply adding more water and or water/soap. This was my first attempt at using a wash and it is pretty idiot proof. I also experimented with the oil and lighter fluid wash. This also worked extremely well.
One thing I liked about the water colors is the control. You can dab a q-tip with the color and dirty a large area. It looks pretty realistic to me. Hope this helps ya!!!!!