For good creative play, they have to limit the diversity of pieces and colours within a single theme. Different sets from the same theme need to have similar pieces in the same colour scheme so they can co-exist nicely. Take these 3 sets from the classic 1990s Pirates, for example:
Combining these 3 on a big baseplate can give you something completely new and interesting that still belongs to the Pirate theme. The bulk of the parts are white and yellow-coloured bricks for walls and architectural elements.