How often does LEGO make colour substitutions? I'm currently building the UCS Millennium Falcon and ran into a step where the instructions say to use two different shades of grey in the greebling, but the bags for that step only contain one single shade (but the right quantity).

This seems odd for such a prestige level release, so my question is whether this is "normal" or whether I should be asking for replacement parts.

To clarify - step 12, the round vents at the top back of the Falcon should contain black 1x2 grills, and dark gray 1x2 grills are used elsewhere on the paneling for detail.  The bags for this step contained no black grills - just the dark gray. Proper quantity, just wrong colour.

This isn't a case of the constructor misusing one part accidentally in place of another - the step is built with no missing parts, only a few random spares, it just has the wrong colour grills in the vents.
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.