>>5268400Basically just to distinguish him from the rest of the Matoran, make him look like an odd man out. They gave him a blue mask because it's a primary color, like red and yellow. Color has always been an important feature in Bionicle, often denoting a character's personality traits and element. Takua's RBY scheme was probably chosen to, 1, represent a lack of nationalism between tribes, and 2, subtly hint at Takua being from a secret 7th tribe.
Of course, by the time that was formally revealed, there were like 20 new tribes with their own elements, and a whole bunch of living Av-Matoran, so Greg decided to retcon Takua into the first Matoran ever to make him seem more special.
