Pliny NH 35.85 Quoted by Lucilla Burn 'Hellenistic Art from Alexander the Great to Augustus' 2004His [Apelles'] politeness of manner made him all the more agreeable to Alexander the Great, who often visited his studio ... but in the studio Alexander would talk a great deal without understanding, and so Apelles persuaded him tactfully to keep his mouth shut, saying he was being laughed at by the boys who ground the pigments..
I find it difficult to believe that this is not apocryphal, partly because I can't imagine anyone telling Alexander to shut his mouth, particularly after he came back from India. It is also difficult to know when he could have occurred as it seems unlikely that Apelles would have followed Alexander as far as Bactria and India. It would seem more likely it occurred in Europe or Asia Minor, but it smacks of the leisured Roman aristocrat visiting an artist with his cronies because he had nothing better to do, and Alexander was far too busy warmongering in Greece, Thrace and against the Persians. Unless he were simply visiting the studio to pose for Apelles or inspect the progress of a painting.