Why a yellow Mercedes? Why not blue or green, or a nice classy silver? Well, some scholars think that the yellow Mercedes represents Hans, the blond European man promised to Macabéa by Madame Carlota, the fortuneteller, and that maybe Hans is even the driver of the yellow Mercedes: a blond European man driving a blond, European car.
Sure. We can buy that, and it would even fit that Madame Carlota would both be right and also tragically wrong.
Of course, on an even simpler level, this symbol is dead easy: Macabéa is run over by a large yellow Mercedes just like the poor are always squashed by the rich. They're not exactly budget cars, after all. And even as she's dying, Macabéa notices that "the yellow Mercedes was truly luxurious" (5.419).
Also, did you notice anyone conspicuously absent from Macabéa's deathbed? Yeah. The driver. He speeds off, without any concern for what he hit.