Complete the list of primary colours: red, yellow, and _____.

Correct answer: Blue

In the traditional RYB art-colour model, red, yellow, and blue are the three primary colours. They are treated as basic colours that artists use to create secondary colours through mixing. Therefore, blue correctly completes the list. Green is made by mixing yellow and blue, while orange is made by mixing red and yellow; black is not one of these three primary colours.