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By the time Veronese arrived in Venice in the 1550s, he was a fully formed artist whose work looked more like that of central Italian painters – Giulio Romano, Parmigianino, Correggio – than it did like any Venetian artist. Generally speaking, Veronese's use of bozzetti or preliminary sketches and preparatory drawings was at odds with the colourist approach of the Venetian school, where the artist typically paints the under drawing directly onto the canvas.

As we see in a thrilling gallery hung with major altarpieces painted in Venice in the 1560s, Veronese did not abandon the firm outlines and smooth surfaces of the early mainland paintings to embrace the scumbling technique used by Titian and Tintoretto. What feels new, as you find in works like the figures he painted for the organ shutters in the church of San Geminiano, is the monumentality and the contrasts of light and shade. And it's not colour Veronese learnt from the Venetians, but tonality. Compare the way colour registers as separate patches of paint in an early work such as The Conversion of Mary Magdalene to the tonal unity you find in his huge altarpiece, The Mystic Marriage of St Catherine. Here, despite the strong blues, rose red, gold and silver, our overall impression is of a picture surface suffused with light.

_{The Martyrdom of Saint George by Paolo Veronese}

Over time, it is broadly true to say that Veronese's pictures become darker, the colours richer and the brushwork freer. But his stylistic development isn't linear, and we have to imagine that he kept both drawings and compositional sketches in his studio and used them to recycle figures, gestures and poses as they were needed.

Precisely because of Veronese's tendency to reuse and repeat figures, this show has its ups and downs. Although his later religious pictures may be very beautiful, they feel like conventional products of the Counter Reformation. As we know from Veronese's famous encounter with the Inquisition, he was an artist who needed to give his imagination free rein. If, in a subject like the Adoration of the Magi or the Resurrection, such invention was out of the question, some ineffable connection between the artist and his subject isn't there. It's not that he paints on autopilot, like so many Roman painters at this time, but that the creative spark is missing.

But when it's there, what a painter he is. You see it in his huge altarpiece from the church of San Giorgio in Venice, which shows the patron saint of England in the moments before his martyrdom. The subject is rare in art, so there was no preordained way to show it. I wonder too whether a commission to depict the beheading of an English martyr may have fired the artist's imagination at a time when Catholic Europe was all too aware that Elizabeth I was about to create new martyrs. Whatever the answer, his performance here is electrifying.

As usual, the viewpoint is from below, so that we see the kneeling saint encircled by his tormentors against a clear blue sky and the vertiginous columns of a Roman building. Heavenly light falls on his face and upper torso, making him the radiant centre of a complex multi-figured composition in which nothing is left to chance. Such a work would have been impossible to create unless the artist had some kind of finished model before he started to paint. It seems likely that he first worked out the composition in a sketch, then made studies on paper of figures, including gestures and facial expressions. There is no evidence that he squared his drawings in order to transfer them to canvas, as Raphael would have done. Though there are no drawings by Veronese in this show, at the time of his death in 1588 at least 1,000 were recorded in his studio.

_{Allegories of Love, Happy Union by Paolo Veronese}

The show rises to its climax in the room in which the National Gallery's Four Allegories of Love are displayed alongside the Metropolitan Museum's Mars and Venus United by Love. It is hard not to feel that in these gently erotic mythological scenes we come close to his heart and soul. No one quite knows what is going on in the four ceiling paintings that make up the series of allegories, but since most of them show nude or semi-nude men and women it is safe to say that they are about the joys and sorrows not just of love, but of physical love. To me they look like impresse, a nobleman or woman's personal device or shield which often took the form of a painted metaphor. But since their titles – Unfaithfulness, Scorn, Respect and Happy Union – were given only in the 18th century, their original symbolism or significance is lost.

That hardly matters as we feast our eyes on the way Veronese deploys pearl grey, violet, rose, lemon yellow, gold and silver against a soft blue background: to step into the gallery where they are shown is to experience something like a whoosh of air as we look up as though through a ceiling to the open sky. Suddenly we find ourselves in an amoral, playful world such as we won't encounter again in art until Watteau, Fragonard and painters of the Fête galante. If that's "mere painting", there's a lot to be said for it.

March 19-June 15, sponsored by Credit Suisse; nationalgallery.org.uk

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