I personally recommend Sard Wonder stick for the stain remover
You can use this method for both synthetic and hair brushes
Moisten stain remover stick under warm running water
Moisten dirty brush under warm running water
Roll brush against the center of the stain remover stick
Continue this under warm running water until all color residue is gone and water runs clean
Lay brushes to dry on a towel overnight
And voila! Clean, soft brushes the next day!