Rosa da Tivoli acquired a wonderful facility in design and execution, for which reason he was named Mercurius by the Bentvogel Society. A remarkable instance of his powers is recorded by C. le Blond, then a student at Rome. "It happened one day," says he, "that several young artists and myself were occupied in designing from the bassi-relievi of the Arch of Titus, when Roos passing by, was particularly struck with some picturesque object which caught his attention, and he requested one of the students to accommodate him with a crayon and paper. What was our surprise, when in half an hour he produced an admirable drawing, finished with accuracy and spirit."

It is also related that the Imperial Ambassador, Count Martinez, laid a wager with a Swedish general that Roos would paint a picture of three-quarters' size, while they were playing a game at cards; and in less than half an hour the picture was well finished, though it consisted of a landscape, a shepherd, and several sheep and goats.