Sorry, I'm not sure if this is the right place to ask this much detail about heraldic blazons.
I'm curious if traditional heraldry allows you to place a charge on top of a variation on the field. The variation of the field needs to follow the rule of tincture, so it must be made of alternating color and metal, which means any charge you put on it, whether metal or color, will violate the rule of tincture.
For instance:
Bendy of six, or and sable. A chief argent.
Whatever tincture you make the chief, it will violate the rule of tincture either because it will overlay the or (metal), or because it will overlay the sable (color).