Why may red-violet appear relatively cooler beside red?

Correct answer: Its blue influence feels stronger compared with red

The governing concept is simultaneous contrast and relative colour temperature. Red is strongly warm, whereas red-violet includes both red and blue influences. When red-violet is placed beside red, its blue component becomes more noticeable by comparison, so the colour may appear relatively cooler. It does not actually turn blue, the background does not physically mix into it, and its lightness or value does not necessarily change.