When a video shows the past or a bad example in full color, viewers can mistake it for the present or the recommended approach.
Use black and white for these moments, then return to color. Viewers can see the change straight away, without extra explanation.
Black and white feels like an old photo. Use it for a story from history or the way things used to be, and viewers feel they have stepped back in time before anyone says so. When color returns, they know they are back in the present.
Taking the color away makes a stressful or painful moment feel heavier, so viewers sense something is wrong before they hear why. When color comes back with the better approach, it feels like a relief, and that contrast helps the lesson stick.