A change from colour to monochrome is immediately noticeable, but its meaning depends on the film around it. Start with the transition itself before deciding that it marks a memory or a dream.
Describe the boundary
Note whether the change happens at a cut, fades gradually or appears within an object such as a photograph. Record what else changes at the same moment: location, sound, speaker or time reference.
Then look for another example. If the film uses monochrome more than once, do the passages share the same circumstances? An early assumption becomes more useful when later scenes can support or challenge it.
Compare a repeated detail
Choose a face, costume or setting that appears in both treatments. Describe what becomes easier or harder to notice. A pattern may stand out differently even when the object itself has not changed.
For a viewing note, separate “the image loses its colour” from “the scene feels distant to me.” The first records a visible choice; the second offers your response. Connecting them through a precise moment gives the interpretation substance without turning one visual device into a universal code that every film must obey.