Never mix paint with your best brush

If you have special brushes - i.e. kolinsky sables or even mixed synthetics with very good points

see pics...

You really will not want to be ruining those beautiful pointy tips.  I can assure you, if you start mixing paint with them, those tips will be no more!    Always mix your paint with a lesser brush, sometimes I use a very cheap oil bristle brush or similar, for mixing paint.  That way it doesn't matter if the brush gets ruined.