Two rules, then diminishing returns

More has been written about wardrobe for professional portraits than about anything else in the subject, and it is close to the least important variable.

Avoid fine patterns

Narrow stripes and small checks produce visual noise at small sizes and interfere badly with digital compression. That is a measurable effect rather than a stylistic preference.

Avoid matching your background

Whatever matches disappears. A shoulder that merges into the backdrop leaves a floating head, which is the same failure as dark hair against a dark wall and just as invisible at full screen.

After those two

Diminishing returns immediately. Colour, fabric, neckline, jewellery — real considerations for a printed portrait, and almost entirely absent at the size a profile picture displays.

The thing the guides do not say

At forty-eight pixels nobody can see your clothes. Not the cut, not the fabric, often not the colour beyond light or dark.

Effort spent on wardrobe for a small-format headshot would be better spent on the crop.