Why the use of different colors for the inner bricks? I'm currently building the set 10221 and it's not the first time I notice that the colour of bricks used for the inner parts (parts that aren't visible) of the set are different from the main colour of the set. For example: the main colour of this set is light grey/light bluish grey but bricks for the inner parts are in blue, red, tan, etc.

Is there any reason for LEGO to use different colours (some are difficult to find if bricks are bought separately) for those sections of the set?
LEGO uses different colors on interior (non-visible) parts of a build to make the building process easier. Different colors (especially contrasting colors) are easier to recognize in printed instructions. There might also be a cost-saving factor involved, as those colored bricks might be in larger supply at the factory and hence easier to source for a given set.