the best thing I have ever used for washes is a blast from the past WATERCOLORS nothing expensive mind you the cheaper the better I use mostly the Rose Art trays, the ones that have the little round "cakes"of paint .Watercolors are alot more forgiving than any other wash medium I have ever tried,if ya goof up just wash it off No solvents,no staining,no fumes,inexpensive & safe to use around kids and pets. Try it out ! TTFN
PS acrylics make for horrible washes once they set up (& they do it FAST!!!) theyre almost totally impossible to alter or remove!