Thank you Lurch_M.

However helpful Lurch's explanations usually are, here they might be a bit misleading for speakers of certain languages. Saying that “a” corresponds to “of” might suggest that it is a preposition; instead it is a kind of article-pronoun, and the “of” is in the genitive of “peronului.”

Speakers of certain languages (such as mine) might have an easier way thinking of “the blue color the one of the platform.” The “a” would then correspond to “the one.”

Lurch, please don't take it personally.