What You Wear Is a Requirement
Clothing has to contrast with the background.
Hong Kong guidance asks applicants to wear regular attire that contrasts with the background, and specifically warns against clothing that blends into it — a white or very light top against a white background being the obvious case.
This is worth taking seriously because it is a real failure mode rather than a stylistic note. If your shirt is the same tone as the wall, the outline of your shoulders disappears and the photo reads as a head floating on a blank field. Automated checks and human reviewers both key on that outline.
Mid-tone or darker colours are the safe choice: navy, grey, olive, burgundy. Avoid busy patterns and logos, and avoid anything strapless or off-the-shoulder, which produces the same vanishing-shoulder effect.