Exception.

An unusual element stands out because its neighbours establish a pattern. If everything changes at once, there is no clear exception—only competition.

Spend attention carefully.

Size, colour, weight and position can all create emphasis. They do not have to work at the same time. A small change in one property often makes a stronger distinction than several unrelated changes.

A useful exception points to useful information. Make the next step stand out, distinguish a warning, or mark the item the reader is looking for. If every item needs emphasis, the underlying order may need another look.