"I can't wear them if they are grey." Gifford notes that “In the mid-Victorian world, the period of a son’s deep mourning for his mother (black suit, shoes, socks, and tie and a sharply limited social life) would have been a year and a day. By 1904 the rules had been considerably relaxed, but Stephen is adhering to the letter of the old law.” We learn in Ithaca that Stephen's mother was buried on 26 June 1903; so only a few more days remain before the old custom will allow Stephen to “go into ‘second mourning’ (gray would be acceptable).”
The extreme rigidity of Stephen’s adherence to the old custom is surely related to the terrible guilt that has made his mother a ghoul in his eyes. To this realistic explanation Joyce adds a symbolic parallel. Stephen’s insistence on dressing in formal mourning attire when others in his world recommend moving on with life likens him to Hamlet. The contrast, later in this episode, between his mourning clothes and the “gay attires” of Mulligan and Haines furthers this resemblance, since Hamlet first appears in a scene of high court ceremony where everyone else is dressed festively to celebrate marriage and dynastic stability, and his mother urges him to "cast thy nighted colour off." Somber black attire marks both Stephen Dedalus and Leopold Bloom on the day represented in Ulysses, furthering a father-son symbolism based chiefly on Hamlet.
By the same token, Mulligan's mocking criticism of Stephen's mourning ritual—"Etiquette is etiquette. He kills his mother but he can't wear grey trousers"—aligns him with those who would threaten this pursuit of spiritual paternity and this commitment to principle. He is Gertrude and Penelope, mothers who stand in the way between father and son. And he is Claudius and Antinous, usurpers whose calls for social normalcy mask political coercion and bad conscience.