How often does LEGO make colour substitutions?
In my experience, this typically means you used the other colored part by accident somewhere else in the model, for example in a place where it is used on its own, and it was difficult to determine the exact color of the piece at that time. This happens a lot with colors like dark bluish gray, light bluish gray and pearl gray. You will need to go back through the manual to find previous places where this piece was used and confirm you used the right color.