Which of the following is considered a warm colour?

Correct answer: Red

The governing concept is the conventional classification of colours by temperature. Red is generally placed in the warm group because it is associated with fire, heat, energy and visual advancement. Blue and green are commonly classified as cool, while violet is often treated as cool, although its character can vary when it leans towards red. Thus red is the only clearly warm choice among the four options in this school-level question.