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I noticed recently that hidden Lego pieces in many sets (I did not check them all of course) have completely different colours than those are visible. They usually differ from not only the colour of the visible parts but they seem to be somewhat randomly coloured. Why aren't they coloured the same as the other pieces? Is it cheaper to use base colours?
Example: The statue of liberty set (21042) practically has two colours visible (green and sand), however inside there are red, yellow, blue and black pieces as well.