Simple rule:
ALWAYS use a wash that will not effect the paint underneath. If the paint is enamel use a acrylic wash.
There are pros / cons to each type. Here is my experiences & humble opinions on each:
Acrylic wash. Over enamel, usually a layer of enamel flat clear like 'dull coat' or 'clear gloss' can go over the enamel paint without clear. You can thin with demineralized water and a drop of dish soap to break the surface tension to allow a better flow. If thinned with water it will take longer to dry, I usually use isopropyl rubbing alcohol.
Pro: Harmless to paint underneath if all enamel.
Con: looks great going on, it can dry with less than satisfactory results. Can form lines that look like cracks on the edges.
*Hint work fast and continuously, better for 'pin washes' or oil / grease stains where a noticeable edge is desired.
Enamel wash. Over acrylic paint or a layer of Future acrylic floor polish. Be sure Future is completely covering the surface and cured for 24-48 hours.
Pro: Flows well and looks as good as initial application once dry.
Con: Due to nature of enamel thinner it is risky in that it can 'attack' base layer. If all under layers are acrylic you are safer to use a enamel wash. Work fast without too much brush contact pressure.
Oil paint. New to me with great results. Used to 'tint' paint underneath that was a tad off hue. Same procedure as enamel washes. I used over Future as pictured below.
Tempura paint. (kids poster paint)Airplane modelers use this over a gloss coat to accent panel lines. It is thinned with demineralized water & a drop of dish soap, applied liberally then wiped off with a cloth moistened in water in the direction of air flow. Due to its temporary nature the entire coat can be easily removed if desired to start over. I personally never tried this technique but will on my next airplane.
* Shown is a tank painted in Krylon 'buttercream' enamel that was not quite the correct color. A layer of Future was allowed to cure then a wash of Raw Sienna oil paint was used to get the correct (close enough) color. The barrel is not done & the hull is at this point.
I hope this helps you with your wash question.
Jason