for acrylic paint
I use hot water an dish liquid.
pour in half a cup of steaming hot water and drop some dish liquid in.
now dip the dirty bushes in and swirl it about.
Take out and use a cloth to dry it
I use the same, but I use toothpaste as well.
I take the dirty bush, swirl it around the glass, remove it and take a pea sized amount of toothpaste and massage it into the brush. Then I resoak it and clean it off with a cloth.
If the brush is still dirty, I redo it