Each matrix has 16 cathodes (8 red + 8 green) so 7 would be chained already. If you connect the cathodes of each color together and do rowscanning in stead of columns, perhaps it could be done with 5?
Nice project 
Each matrix has 16 cathodes (8 red + 8 green) so 7 would be chained already. If you connect the cathodes of each color together and do rowscanning in stead of columns, perhaps it could be done with 5?
Nice project