what is the significance of Sonia having a yellow passport

You actually have to do some research on this one but the general consensus seems to be,

"A yellow ticket" is a yellow-coloured document (an alternative form of a passport ID) which enabled a woman to be legally engaged in prostitution. So that means it was acceplable (in Russian, "bearable", "tolerable") and that's why brothels were called "tolerance houses".

As well Dostoevsky uses the colour to denote aged, decaying, damaged goods, claustrophobic, contaminated, wasted, ugly, vulgar and gaudy.

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