Attention, memory and perception
Von Restorff effect
Also called Isolation effect
The item that looks different from the others around it is the one people notice and remember.
Among similar things, the odd one out stands out. That's how a primary button, a highlighted plan or a warning gets seen.
Use it sparingly: one emphasised thing per screen. When everything is highlighted, nothing is.
In practice
A pricing table where one plan has a coloured border and a 'recommended' label draws the eye first.
Common mistake
Relying on colour alone to make something stand out, which fails people with colour blindness. Add shape, size, a label or position too.
Where it comes from
Named after Hedwig von Restorff, who described it in 1933 from experiments in which an item unlike the rest of a list was remembered best.