perception
Similarity
Things that share a visual trait – shape, colour, size – are perceived as the same category, even when scattered across the screen.
This is why every primary button in a product shares one colour and every secondary action shares another: the sameness itself is the signal. A user who has clicked one accent-coloured button on this site knows what every other accent-coloured element means without being told.
The flip side is an honesty obligation: if two things look the same, users will assume they behave the same. A link styled like a button, or a heading styled like a link, breaks a promise the visual system made. Consistency isn’t aesthetic tidiness – it’s the mechanism that lets users transfer learning from one screen to the next.