Is there a particular way to manipulate a grain bill to change the color of the head of a beer? Ie if you want an off-white head in an RIS as opposed to a brown head?
I think you want red food coloring. This will definitely give you a pink head and won't be noticable in the dark stout beer itself.
I do this every year with green coloring on St. Patricks day and it works great!
A few drops per 12oz beer should be good.
I don't understand the chemistry behind it, but its pretty straight forward. Roasted barley is unmalted, where chocolate or black patent has been malted. When using roasted barley the head on a stout stays tanish in color. When you rely on the malted versions of dark roasted grains they contribute darker colors to the head. Think the difference between classic dry stout (all roasted barley) and a classic English Porter (primarily black patent and chocolate malts). The dry stout has the off white head, where the porter carries that browner chocolately head.