Pink rose to prominence as a regal colour denoting the aristocracy and nobility. This colour was chosen for Goku Black because Goku Black, who is Zamasu, is the Supreme Kai of the Universe 10. Thus, he is part of the Divine aristocracy/nobility.
Pink first rose to prominence as the favored color of Madame de Pompadour, a mistress of Louis XV. A specific shade of the color was created and named for her by the Sèvres porcelain company, who enjoyed her favor. Their fine porcelain was a prized status symbol in the 18th century, and this rich hue of pink came to be associated with the opulence of the royal court and all its trappings, though it was not assigned to a particular gender.
https://www.artandobject.com/news/color ... al-history
It simply makes sense to use this colour, which rose to prominence in an aristocratic setting, to identify someone who is indeed part of the aristocracy. Since the Supreme Kais are essentially the rulers/overseers of their respective universes.
The point of choosing this colour is that it objectively beautiful, elegant, no one looks at pink and thinks "ew, what an ugly and lowly colour". You would never see a peasant or a commoner wearing pink. So it fits the vainglorious and self-centred personality of Zamasu. And that's why Black's first remark upon using this form, is that it looks "beautiful".
Although you can indeed argue that this colour was used to indicate the start of Zamasu's fall from grace. When he decides that mortals are evil and need to be destroyed, so when he starts to become a villain, the petals around him change from white to pink. So it is definitely used to indicate malice too.