1) Have meeting where you establish the walls should be painted this specific shade of blue. 2) Document this in an email summarizing the meeting and the todos 3) Start painting the walls blue, maybe even finish 4) Toxic Boss comes back and wonders why the walls aren't green. Apparently, they had been looking at the color swatches while sitting under a red tanning light, and expected for you to know what they meant. 5) Go back and paint the walls green. 6) The toxic boss now looks at them and wonders why they aren't red. And you need to paint the ceiling black, because they had a meeting with a potential client at a trade show, and black ceilings are the new hot thing. Clearly, you can fit all of this painting, and repainting, into the original timeline (despite your protestations), because the toxic boss told the client you'll be shipping it by date.
No amount of documentation works here. When you point out to the toxic boss your original agreement, they look puzzled at you, wondering why you aren't team player, and why you aren't listening to customer needs enough. There's another team at work who paints signs, and they are fine with daily changes.
You talk to the toxic boss about making a process for iterating on this and documenting expectations better, but then they say you're too rigid and process will slow things down- they are an idea person, and just like thinking about colors, not the details.