I use three brushes -
1) One brush for black and color 8
2) One brush for medium browns
3) One brush for light browns (ravello/whiskey)
Thank you very much for the tip. I am indeed using separate brushes. I use 1 for color #8, one for browns and one for lighter colors. However, I do think it is an interesting topic to if polish residue will transfer on brushes. My understanding is that the primary purpose of the brush is to remove any excess residue, dirt, etc so why then would it add residue back to a finish?
Trolling ebay for a friend found this, current bid $255 just under 3 days left.
And these for $299.99 or best offer.