Remember what Hui Mun Keung dressed like in his early days in Shanghai, when he was running that nightclub with Ah Lik: usually a light grey suit with a red necktie. Symbolism: Mun Keung was an independent man, so he dressed as he liked, and the colors were light because he was working towards a bright dream at the time.
When he worked for Fung Ging Yiu, Mun Keung ditched the straight ties for bowties and the gangster-style suit for more of a respectable businessman's suit. He even combed his hair differently (again, more like a respectable corporate businessman than an overt gangster). This is because during this phase of his life, Mun Keung had to function (at least publicly) as a respectable businessman (because Fung Ging Yiu's organization, at least on the surface, was a respectable and legitimate business enterprise). So Mun Keung had to dress accordingly. NOTE: after Mun Keung leaves Shanghai, Ah Lik begins to dress in this manner, suggesting that he has taken over Mun Keung's old role in Fung Ging Yiu's organization.
When Mun Keung returns to Shanghai, he's dressed very similarly to how he did when he was an independent gangster in the early episodes (including the slicked back hairstyle). The one difference was the COLOR of the suits. Whereas the earlier Mun Keung wore greys and beiges, he now wears BLACK...showing that he has returned to where he started, but with a darker, crueler edge.