Here’s the scan from a roll of Fp4 black and white film I shot. The purpose of this was to test the formation of magenta dye during development and retained after blix. The thing to note here is that this dye doesn’t “paint” the silver like what I’ve seen others try. The dye is colorless and forms directly at the location of the silver. This is also a very short lived reaction, important to keep the dye from migrating layers when used in Kodachrome.
Here’s a way to think of this.
A man has a peice of metal and he dunks half of it in water, where the metal was in the water he observed that it rusted.
The man has a recipe for a blue dye that needs three ingredients, 1. Water 2. Chemical “A” 3. Rust.
The ingredients are mixed together but the rust is left out, leaving the man with what looks like a bucket of water.
The man dunks a new peice of metal into this, and as the metal oxidizes, it forms rust. The blue dye begins to form only where the metal has rusted and not in the water itself.
He repeats this experiment but instead uses a waterproof stencil placed on top of the metal. The man observes that the metal is only blue where the stencil was exposed.
This is actually backwards (developer oxidizes, not the film) but it’s the best way I can explain my process.