You can use their dark wash over Tamiya acrylics with no problem whatsoever. They actuallly recommend using their products over Tamiya paints. I use them all the time over my Tamiya-painted tanks.
Their pre-mixed washes are of an enamel-carrier type. Regular mineral spirits will remove the excess. MIG’s washes take a little longer to dry, and dry to a semi-gloss finish. Most people though would tell you not to use black/browns for panel lines on cars; I’ve heard that a darker tint of the body color is actually best.
You could also save your money and just use the thinner you clean your brushes in. I keep a jar of brush cleaner on the bench and when it get dirty I just put it next to the clean, new bottle to use as a wash for panel lines, finned valve cover, chrome emblems or anything else that needs a little “highlighting”. Just touch the brush tip to whatevert you need highlited and the thinner runs off the high places and collects in the grooves for a dramatic effect. Works really good on factory style wheelcovers from the fifties and sixties and grill bars too.
i asked a similar question in another part of this forum. i have been using Mig pigments on my car and truck builds for a little while and really like them. for panel lines on alot of my builds that are of weathered subjects i use thinned Testors “rubber” color, which is a dark flat brown. it seems to have been working pretty well for what i’ve been doing, but i’m learning more here about different color washes and now even filters which i find very interesting.