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?